Through His Everything.

Indirect Characterization. Anyone who has read this book knows which character is developed indirectly-Jake.
Every new character we meet is in a scene involving Jacob, so we see how people interact with him. The author also reveals Jake's character to us through his own thoughts and feeling-his narration.

Through Cohn:We see Jake's somewhat boring personality. He is uninspired by anything except for work. Take this conversation:
" 'Would you like to go to South America, Jake?' [Robert] asked."
"No."
"Why Not?"
"I don't know. I never wanted to go. Too expensive. You can see all the South Americans you want in Paris anyway" (page 17).
I mean seriously, how boring is that? But because Robert wants to stray away from his mundane life, Jacob's bland personality shines through.

Through Brett: Now we get to see Jake's sentimental side. "The street was dark again and I kissed her. Our lips were tight together ..." (page 33). After Brett is introduced, Jacob becomes an entirely different character. His tone completely changes when he is with Brett and he isn't just the cynical character we knew him to be.

The same thing happens through Frances. Because she confided in him about her problems with Robert, we know that he must scream "I'M A SENSITIVE GUY; COME TALK TO ME!". Even though it might make him really uncomfortable, it's just his aura. Now I know he really is a nice guy.

Finally, we can also learn a lot about Jacob's personality through his own thoughts.
"It is very important to discover graceful exits like that in the newspaper business..." (page 19). Cohn is really pessimistic about everything (except towards women); his work is his excuse out of almost everything fun. Even though he does tend to party ( a lot) he never seems to really enjoy himself. "I have to work in the morning,' I said. 'I'm too far behind you now to catch up and be any fun" (page 41).