The existence erasure doesn't work on people important to the plot, so it doesn't work on Ritsuka. Ritsuka summons almost any servant and they one shot and/or hax him to death. Shakespeare breaks his psyche, Anderson amps protag, Shiki MEODP, King Hassan deaths, Musashi just does Zero stuffs, Gil Enuma Elishes, Cu Gae Bolgs, Maou Nobu erases his existence due to his mysterious nature there are so many options and possibilities to just one shot Prota Kun and keep him down. So if Ritsuka gets off a summon, there is nothing Prota Kun can do. Servants resist everything Prota Kun can do, except Plot Manip, and they have so many ways to circumvent that or just one shot and keep him down. Meanwhile, if Ritsuka doesn't get off his summons, neither have calcs, so baseline 9-C for both, Prota Kun outskills, but Ritsuka has Gandr which one shots but doesn't prevent ressurection. So Ritsuka would eventually go for the talk option and Social Influence Prota Kun, and because Ritsuka resists Transmutation, he doesn't get Body Pillowed. So yeah, I'd say Ritsuka. Prota-Kun needs to put Ritsuka down before he gets off summon, magic, or talk. But summoning and talk are Ritsuka's first move.
What does Prota-Kun start with then?
