Spoiled-marked one glaring detail, whereas the self-explanatory or obvious reveals are in plain view.

1) He's an amphibious human (natural habitat being the sea).
2) He's very emotional, to the point it breaks him and drives him to death. While his mind races a lot, he's quiet on the outside.
3) His teriques is black. The teriques is essentially a projection of one's soul. So, yeah. His soul is black. And it bites.
4) To reflect his teriques, his true name (given in Ferines culture by the village chief) is Delques and means "black wings." Besides black, "del" also means fatal, deadly, final—and, in its base form, gravestone.
5) He used to wear this as a disguise, but he's so prejudiced against land folk that he hates it.
6) In canon, an NPC named Clara says: "You misunderstand Walter. He is rude in order to hide his kindness." Ain't she a riot.
7) His "magic" is known as eres and doubles as an intimate connection to the sea. Which is just as racist as he is.
8) Walter is deeply spiritual and selected to use the Whisper Crystal, which is powered by prayer to control over 200 soldier dolls.
9) In his childhood, he was tasked to be Shirley's personal guard. He literally believes that this is his only purpose in life, because with 4,000 years' worth of hope pressuring him to do it right, what's a little boy to do.
10) He almost got recruited into this little musical police group.
11) He flops.
12) Headcanon: He's a Scorpio.
13) Headcanon: The tassel hanging off the left orb on his cape denotes his rank in canon, that being the Merines' chief guard.
14) Vatheon: He suffers from severe existential crisis. Consequently, his instinct is to reject everything.
15) Vatheon: He stopped growing because he's dead, so he's forever 5'7" tall.