Xerneas is a member of Kalos' Vitality Trio alongside Yveltal and Zygarde, and within said trio is the embodiment/deity of life, creation, and growth, and aligned with Souls of Silver. He has two alternate personalities - Xylander, who is wise and generally mature and polite, but has a self-righteous streak born of a subconscious "need" to be the better mon in an argument; and Xander, who's mischievous, irresponsible, and fond of terrible puns, but also much more carefree and a tad more lenient in conflict unless thoroughly angered.
In addition to his usual form of a massive blue stag he can disguise himself as other creatures; his most common disguise is another deerlike mon or a human. As a human in his Xylander persona he's tall, slim, and androgynous while in his Xander persona he's short, squat, and imp-like.
- Xerneas has a close sibling bond with Yveltal, though dynamic depends on formers' persona; in his in his wiser and generally more polite Xylander persona she very much appreciates him being more mature and responsible and not constantly teasing her but greatly disdains his "need" to have the upper hand in an argument, whereas in his more impish and mischievous Xander persona she finds his irresponsibility plus his teasing and trolling towards her thoroughly obnoxious, but finds him vastly preferable to deal with in a disagreement or when otherwise angered due to his more lax nature. Both love and care for each other regardless, though.
- Arika considers Xerneas a father figure due to being the human incarnation of one of his creations, the Lunar Pendant, and refers to him as "Legendary Dad". Xerneas has fully accepted this, and they get off mostly well regardless of his persona - especially due to Arika's own multiple personality issues.
- He has a rather enigmatic connection with Lifealope, who refers to him as "cousin".
- He is rather fond of Fool and drinks tea with her often, though they got off on a rocky start because of Solana's Obscuric Life creations being made without his consent (Though she now has a permit).
- He has a great fondness for tea, and many of the plants he grows in his grove are specifically for different brews of tea.