Kaseto's colors allow you to play extremely powerful Blue cards such as Cyclonic Rift, plus all of the counterspells and other tempo options available in the color. Having backup countermagic makes building up your army so much easier and alpha strikes become much easier. Also, now you can play three of the best Snakes in the game in your Snake Tribal deck: Coiling Oracle, Lorescale Coatl and Mystic Snake. Helpfully, all three of those were included in the same Commander deck as she. You can also pack in all the Blue "good stuff" that you like! Ophidian, a mono-Blue Snake that draws you a card when it isn’t blocked, works well with Kaseto, as well.
With Seshiro the Anointed and the other Legendary Snakes all being pretty good on their own, Snake Tribal becomes a lot more competitive. There is also an enchantment called Xenograft that gives all of your creatures a chosen creature type. By setting it to Snake, you can play any Blue or Green creature you like. These include powerful Legendary Creatures such as Momir Vig, Simic Visionary, or utility creatures such as Master Biomancer or Murkfiend Liege. You can also play any Blue or Green Changelings that you’d like - such as Chameleon Colossus - as they count as all creature types at all times.
In the 99, Kaseto seems like a good option in any deck that you want unblockable creatures. Blue/Green/X or 5-color decks that play big creatures could benefit, even without the extra +2/+2 boost exclusive for Snakes. Kaseto is best played as a Commander, however, as she screams “build around me.”
Get suggestions on what cards to use in your Kaseto, Orochi Archmage deck from EDHREC.