I'd asked people who they'd most like to see next for this, but everyone gave different answers! So I added my own personal vote into those that had been cast by you guys, and we got Seto Kaiba for the next subject.
Before I move on to Kaiba though, I wanted to try and figure out just who is the oldest and the youngest out of the school-going Yu-Gi-Oh cast. In this document (PDF/Adobe Acrobat), it is stated that the cut-off for entry into Japanese primary/elementary school is April 1st. Thus, someone that turns 6 years of age prior to April 1st of a given year is allowed entry into Japanese primary school, while anyone born AFTER that point is made to wait until the next year. If we stick with the theory of the cast being born in 1980/1981 (speculative; in Vol. 1 of the manga, they're shown to be in class 1-B, aka 10th grade; by Battle City, they are all age 16 and in class 2-B, aka the 11th grade), then the age roster is as follows:
Italicized names are those whose ages and/or birth dates can be approximated, but are not the same age as Yuugi and Co. The italicized names are those of characters whose birthdays (month and day) AND ages were explicitly stated, rather than their birthday and grade (as it was for the main cast). Chinese zodiac signs are in parentheses.
Keith Howard - August 12th, 1969 (Earth Rooster)
Pegasus J. Crawford - October 8th, 1971 (Metal Pig)
Mai Kujaku - November 20th, 1972 (Water Rat)
Isis Ishtar - April 5th, 1977 (Fire Snake)
Ryouta Kajiki - March 1st, 1978 (Earth Horse)
Espa Roba - April 1st, 1979 (Earth Sheep)
Hiroto Honda - April 19th, 1980 (Metal Monkey)
Yuugi Mutou - June 4th, 1980 (Metal Monkey)
Anzu Mazaki - August 18th, 1980 (Metal Monkey)
Ryou Bakura - September 2nd, 1980 (Metal Monkey)
Seto Kaiba - October 25th, 1980 (Metal Monkey)
Malik Ishtar - December 23rd, 1980 (Metal Monkey)
Katsuya Jounouchi - January 25th, 1981 (Metal Monkey)
Ryuuji Otogi - February 28th, 1981 (Metal Rooster)
Dinosaur Ryuuzaki - December 29th, 1981 (Metal Rooster)
Insector Haga - July 21st, 1982 (Water Dog)
A bit of a trick: if everyone of the main cast (non-italicized) was the same age (15) during Duelist Kingdom, then that means the tournament fell sometime between February 26th and April 17th, 1996. (Japanese law says that you age one year on the day BEFORE your birthday, hence the subtraction of 2 days from Otogi and Honda's birthdays. I include Otogi's age and birthday even though he wasn't involved in Duelist Kingdom, because he's the same age as them, and was, for a short while, in the same class as Yuugi and Co.)
The thing that screws everything up is the fact that the character guide uses the ages of all the characters as of Battle City (except Pegasus and Keith Howard, because they "died" during/after Duelist Kingdom, so their ages are their ages during their first appearance in DK), which was the arc running during the guide's printing (in 2002). Like Duelist Kingdom, the tournament itself was shorter than the run of the manga, so publication dates can no longer be used to approximate ages and/or birthdays.
This is also why technology like cell phones was seen in Battle City (by the early 2000s, it would make some sense to have such technology), but why it doesn't fit if you assume the birth dates of the characters are as they appear above. Like many manga-ka, Takahashi probably thought of Yu-Gi-Oh taking place in real-time, even if that wasn't the least bit possible (seriously, since when do duels take several MONTHS?). It is also unknown what the gap is BETWEEN the major arcs (Duelist Kingdom to Battle City, Battle City to the Dark TRPG); this also affects people's possible ages, because it could have been a few months up to nearly a year.
But again, if all the major characters were in the same grade by Battle City, then the tournament had to have taken place on or after April 1st, 1997. (This assumes that Japanese high schools cut-off entrance the same way their primary schools do, and on the same day). It is not known whether the characters are all the same age during Battle City, but that dating helps us figure out the approximate birth dates of the other characters... sort of.
Mokuba Kaiba, while of school-going age, is very difficult to pinpoint, because unlike the others, his age nor grade are never stated in the manga. The only clue we have is the difference between his age and Seto's, and I've seen both 5 years and 6 years. However, he *IS* in primary school (grades 1-6), so that DOES mean he is between 6 and 12 years old, with the most likely ages being 10 (5 year difference from Seto) or 11 (6 year difference). He was born July 7th, which means he started primary school when he was 7, rather than 6.
Seto Kaiba: October 25th (Scorpio). Like Yuugi and the other characters, we can't know Seto Kaiba's exact birth time, but based on the information above, we can at least speculate on what YEAR he was born in, which gives us critical information about his zodiac sign, decanate, and other facets! We can guess his ascendant based on the personality traits of the planets that traveled through Scorpio on October 25th.
In the year 1980, Scorpio ran from October 23rd, 6:18am (Greenwich Mean Time) until November 22nd, 3:40am (GMT). If we assume that all the main characters (like Kaiba) were born in Japan (-9 from GMT), then Kaiba's birth time would have had to have fallen between 10/22/80 9:18pm and 11/21/80 6:40pm-- which it does. The timing (and adding and subtracting for GMT) isn't as important for characters whose birthdays fall directly into a sign, regardless of time conversion; it's more important for characters like Mai and Honda, whose birthdays fall on a cusp. So when we get to them, I'll get more in depth with this. :)
Because Seto is born so close to the start of the Scorpio sign, he is a first decanate, of the 2nd degree. These beginning degrees of a sign are very powerful, and relate to the body, or the Physical aspect. The closer one is to the 0th degree, the more they are related to beginnings; the closer they are to the 9th degree, the more they are related to endings, and physical things. This is a decanate of people who are materialistic, sensible, and practical. This also means that Kaiba's decanate sign is the same as his sun sign: Scorpio. That explains why many Scorpio aspects are "doubled" for him, and can be more extreme than with other Scorpios (such as Mai, who is a third decanate Scorpio/Sagittarius cusp, with Cancer as her decanate).
Scorpios born during this time have an exceptional affinity for games and puzzles-- which is no surprise, considering how dedicated Kaiba is to winning "games" at all costs. To him, they're not "just" games-- they're his life. He takes them seriously, and enjoys the challenge. However, his dedication, as seen in series, can also be costly, as it overwhelms him and overshadows other important factors in his life (family, a social life, libido, etc). However, it's helpful in a variety of positive ways, including a keen business sense, an awareness of the world, and the ability to interpret clues in a logical fashion. The danger is becoming obsessed with petty details or losing sleep over something one can't change.
Scorpios around this time are also incredibly blunt and honest, and don't hesitate to tell people when they think they're bullshitting them-- as Seto as said to Yuugi (and Jounouchi, and Pegasus, and Dartz, and...) on a number of occasions. Scorpios of 1980 need someone who can handle their blunt attitude and direct approach; other people that are faint of heart and can't take an honest opinion won't even appeal to someone like Kaiba. (Hence, someone who DOES stand up to his attitude would probably surprise and intrigue him.) Ironically (maybe?), other "sense-signs" like fellow Scorpios (Mai), Capricorns (Malik), and Taureans (Honda is on the cusp of this sign, but he's a declared Aries) would make good partners, while Cancers (Yuugi), or Pisces (Ryouta Kajiki) can be good friends (Cancers can also be good partners, with Venus in Virgo; see below).
Taking the romantic topic a bit further, during this time, Venus was in Virgo, which indicates that Scorpios of this period are very detail-oriented, and like to do the best they can at everything. Thus, Kaiba would be attracted to someone else of a similar mindset-- someone who is trying to be "the best they can possibly be." (It's not as if "someone else trying to be #1" is an immediate off-putter; it would depend on WHAT they are trying to be the best at. If it's not the same area that he's involved in, he might not be upset by their attitude; if they were, he might see them as a challenge or an annoyance before romantic interest might spark.) And aside from being the best, someone who is detail-oriented would have to be well-groomed, healthy, well-mannered, and intelligent as well (showing that their desire to be the best applies in all areas of life).
As mentioned earlier, Scorpios during this time can take things to the extreme, but they are constantly aware of things around them, and health DOES play an important part here. Even if we don't know what's underneath his trench coats, the speculation saying Kaiba is pretty well-built probably isn't too far from the truth; Scorpios during this time (with Venus in Virgo, the sign of the detail-oriented) are conscientious of their health and diet.
Kaiba also had Mars in Sagittarius during his birthday; this means Kaiba has a great deal of energy, and he typically knows how to expend it by channeling it into his many talents. He is very good at optimizing his time and resources, which could explain how he's a genius, businessman, brother, and top-class duelist, all at the same time. Kaiba's also pretty persuasive and influential, and he's able to see "the big picture." Ironically, the sign is indicative of someone who KNOWS it's not worth it to spend time obsessing over past mistakes or missed opportunities, but the sign in general (double Scorpio) would explain why Kaiba HATES losing, and sees games as much more than a hobby or pastime. Kaiba is especially good at starting things-- but not always at finishing them. Kaiba's an explorer-- though things hold interest, he needs time, energy, and resources to explore them fully-- and sometimes an overwhelming amount of any or all of those three things can mean moving on from something faster than others would like.
Kaiba had Jupiter in Virgo during his birthday; though he's said that he doesn't believe in luck, there's something akin to that working for him in his sign. Kaiba, however, would attribute circumstances working out in his favor as being the result of his practical decisions and careful care for the well-being of others (Mokuba). This also is where he gets his problem-solving skills from; nothing is too daunting for this puzzler! Kaiba makes a lot of things look easy, but that doesn't mean they ARE-- it's just the amount of attention he puts into it that we don't see. Kaiba believes in being prepared for almost every circumstance (hence the metal briefcase with rare Yu-Gi-Oh cards, ready for trading... or other things), and that diligent effort will produce rewards, rather than "luck" or "the heart of the cards." This also means you take on a lot of work that you may not need to --because people know and trust you to do it the way you want, the best possible way it can be done. If Kaiba were to ever get a social or romantic life, he'd need to learn to trust others (Mokuba; Kaiba Corp. employees) to do the job, even if he knows he could have done it BETTER.
(The birth chart I made up for Scorpio, linked below, shows the position of the planets on Kaiba's birthday. Since it's an assumed ascendant, the position of Mars differs from the above, because Mars was in transition between October 23rd and the 25th. But the other planets listed above -Jupiter and Venus- are accurate, and you can see the positions of the other planets, the sun, and the moon, in the chart below as well.)
Ascendant? What ascendant? As previously mentioned, we don't know what time Kaiba was born, but we can guess, based on what personality traits the ascendants of that period have. (The times given below are for Tokyo, Japan on 10/25/80)
-Aries (aggressive, competitive) 3:30-4:44pm
-Gemini (high-strung, clever) 6:20-8:14pm
-Leo (fond of power/command, quick-tempered, forceful) 10:35pm-1:04am
-Scorpio (enigmatic, determined) 5:55am-8:19am (I made a natal birth chart assuming this was his ascendant. Check it out!)
-Capricorn (responsible, serious) 10:40am-12:39pm
-Aquarius (creative, eccentric) 12:40pm-2:09pm
Moving onto blood type, Seto Kaiba has type A blood.
From Bella Online: While outwardly calm, they have such high standards (perfectionists) that they tend to be balls of nerves on the inside. Type A's are the most artistic of the blood groups. They can be shy, are conscientious, trustworthy, and sensitive.
While I personally wouldn't call Kaiba 'shy' in any sense of the word, that can be the impression he gives out to others, considering he tends to be a bit of a recluse while he's working, dedicated to one project. His obsessiveness for detail can mean that he spends a lot of time constructing projects, nitpicking possibilities, and doing work that should otherwise be relegated to others-- and thus, no one really sees him, or hears from him about what he thinks of others. He's definitely trustworthy though, and if the way he reacts to Yuugi and Co.'s overtures is any indication, he IS sensitive- though not "crying at a chick flick" sensitive.
Those with type A blood are supposed to be compatible (not in the donor sense!) with type A and type AB blood holders. In Yu-Gi-Oh, those are...
A: Hiroto Honda, Pegasus J. Crawford, Insector Haga, Espa Roba
AB: Ryou Bakura, Yuugi Mutou
In my book "Your Birthday Sign Through Time" (Atria, 2002), birth dates also have associated "mystical cards," like the ones you play Poker with. Kaiba's is the 10 of Hearts, which ironically is the card of true understanding of love. Likely it refers to a Kaiba prior to the death of his parents and his adoption by Gozaburo Kaiba. Part of him may still be aware of the power of love, as Mokuba knows without doubt how much his brother cares for him, even though Kaiba can be ignorant and even cruel at times. Still, with all the giving and doing Kaiba does, it's no wonder he's a recluse, and lonely-- he doesn't have anyone in his shoes to comfort him, or anyone that he can trust enough to do what he does on a regular basis. For that reason, people see him as extremely selfish.
Closing up with more Chinese Astrology, Kaiba is like Yuugi-- a Monkey; a Metal one to be precise. Look at Yuugi's entry for a description of people born in the Year of the Monkey in general. Metal Monkeys are passionate about everything they undertake: determined, ambitious, and set for success. Anyone with the Metal element has a firm resolve and strong sense of will. Metal Monkeys are also loyal, witty, sophisticated, hardworking, and practical.
Who should be next?
Who should be the next character featured for Astrology 101?
Pegasus J. Crawford
One of the Ishtars (specify which one in the comments!)