Xeno series

The Xeno series is the unofficial name for Xenogears, Xenosaga and Xenoblade Chronicles.

Common Themes
Despite being a metaseries that spans three different publishers (Square Enix, Bandai Namco and Nintendo), the Xeno series has multiple themes that are "typical" for a Xeno game.

Mechs
Each game has its own kind of Mechs:
 * Gears in Xenogears
 * E.S., A.G.W.S. and A.M.W.S. in the Xenosaga series
 * Mechon and Face Mechon in particular in Xenoblade Chronicles
 * Skells in Xenoblade Chronicles X
 * Artifices in Xenoblade Chronicles 2 and Torna ~ The Golden Country

The Zohar
The Zohar is a powerful object that appeared in both Xenogears and the Xenosaga series. In Xenoblade Chronicles 2, an object called the Conduit was introduced that looks almost identical like the Zohar and has about the same attributes. The Conduit is also canon to the original Xenoblade Chronicles. The only game that is still not connected to the Zohar is Xenoblade Chronicles X. Despite that, the Lifehold has the same shape as the Zohar, so do Mythra's and Malos' Core Crystals.

Characters with split personalities
The Xeno series has a long history of multiple personalities sharing the same body in some form. Excamples include Fei, Lacan, KOS-MOS, Jr., Nigredo, Fiora, Klaus and Pneuma.