It's strange, but after meeting them young and having been restored (a little dash of green in the red), it didn't occur to Chara that the other was continuing to pay the price. They switch to sign, addressing their other self.

[Yes and I am constantly surprised you are that] They don't know the fancy word for it, or even the actual word so: [Thick.]

Honestly they're surprised they're even touching him, but they remember their younger self reaching out for a handshake even if the memory, like all from that time, makes their skin crawl. Perhaps that's not universal. Perhaps they're the horrible aberration here. (There's pride, yes. And something else rather more unpleasant.)

Chara smiles like swordblade as their head and upper body turns to Gaster. [Do what?] Nothing they say will not be the truth.