Character Clues
Character Analysis
Speech and Dialogue
Since 99.99% of the narrative is conveyed via direct speech or thoughts attributed to a particular character (with the exception of the chapter intros, in which no characters are present), it's safe to say that the lion's share of characterization in The Waves occurs via speech and dialogue. The characters are super eager to tell us all about themselves, present their deepest likes and dislikes, and contrast themselves with the other characters (see also "Thoughts and Opinions").
Thoughts and Opinions
Basically all characterization in the novel takes place via quoted thoughts and dialogues between the characters. Though the characters are pretty self-centered and fixated on their own reactions to things, they also end up telling us quite a lot about other characters in the process, frequently contrasting their own qualities with those of their buddies. All information about Percival, a central figure in the book and the only main character who isn't one of the six narrators, comes out via this form of characterization.