Idk rate this guy what ELSE do I tell you.
Yuuuupppp I'm apparently making a Polish and Macedonian Alphabet Lore and this guy right here'll be the Protagonist for both So a rundown, YES he's an adult and isn't a coward like basically (almost) EVERY SINGLE AL PROTAGONIST KNOWN TO MAN He's actually a relaxed person who takes the mixed role of B and M, but also N's. J's problem is actually that he's too lax(aka careless) about situations, and he begins to realize this when he realizes some of the letters he still was friends with weren't present at all, and many letters being killed. Thus, his COWARD equivalent would be NIEDBAŁY/НЕВНИМАТЕЛЕН. At the last set of episodes(how many is up to YOUR interpretation wkwkwkwk), he'd begin to save many people as possible from either getting killed, or rarely, kidnapped. He wouldn't be able to save somebody like Y, leading for him to act recklessly and attack the villain, but just barely lose. This results in him being kidnapped for sometime, before eventually gaining loose after seeing a failed attempt to beat the villain, and so gaining all the gems to save everybody from the chains and then defeating the villain.