Print-first characters
Eighty-five manga characters are on this page. It overlaps the anime roster heavily, because most of these series were adapted — but the tag leans toward characters who are best known from the page: Guts, Griffith, Thorfinn, Askeladd, Jolyne Cujoh, Giorno Giovanna, Madara Uchiha, Makima and Frieren.
If you are looking for a specific arc, the manga side is usually the longer one. Several of these characters have hundreds of chapters of material behind them that no adaptation has reached.
Berserk supplies Guts and Griffith, still the reference point for how far a manga will go with a protagonist and how thoroughly a rival can be written. Vinland Saga supplies Thorfinn and Askeladd, whose relationship is the argument the series is actually about. JoJo's Bizarre Adventure spans four entries here — Jotaro Kujo, Joseph Joestar, Giorno Giovanna and Jolyne Cujoh — which is enough to run a conversation between generations.
Chainsaw Man's Makima, Tokyo Ghoul's Ken Kaneki, Bungo Stray Dogs' Osamu Dazai and Chuuya Nakahara, and Naruto's Madara Uchiha and Kakashi Hatake round out the antagonist and rival side. These are characters whose appeal depends on holding a position under pressure, which is exactly what a long chat tests.
Manga is a dialogue-heavy medium with no voice acting, so a character's personality lives entirely in word choice and pacing. That transfers to text unusually well. Askeladd's deflection, Dazai's evasiveness, Senjougahara's precision and Reigen's improvised confidence are all effects you can only get in writing — an animated version adds performance, but the writing is the joke.
It also means a conversation can go where the panel could not. Ask Guts about something quiet, ask Griffith a question he would never be asked, put Frieren in a scene a thousand years after anything you have read. The character keeps their voice; the situation is yours.
The roster is not all seinen. Komi Shouko, Yui Hirasawa, Taiga Aisaka, Hitagi Senjougahara, Holo, Emilia, Rem and Hestia sit on the other side of it, and those chats work on a completely different rhythm — less argument, more continuity. They are also the characters most people come back to across sessions, which is where conversation memory does the heavy lifting.
Isekai is well covered too: Rimuru Tempest, Emilia and Rem carry the reincarnation side, and if you want the genre itself rather than one of its cast members, the narrator and system characters on the RPG tag will build a world around you and let you play the transported lead in it.