Today I replayed Chapter 1, Ralsei acts very differently this chapter, specially regarding Susie.
Considering at the end of Chapter 4 he mentions "I thought if we were kind enough, things would have been different" makes me believe during Chapter 1 he's putting up a facade to try teaching Susie to be pacifist, he even gets pissed off when she's being unreasonable, something we don't ever see him do again.
This also means he knows Susie is not one of the heroes, if his objective is to save her from her fate of being the dragon