Papyrus's Personality remains mostly the same, staying merciful, optimistic and confident in both him self and others. He still has these traits, but they are pushed by Gaster, specifically his mercy. He always hope Gaster will accept mercy. Just like Gaster and Sans, his eyes glows depending on the mood, it can be purple when his fearful, it can be green if his happy and so on.
He is very attached to Sans, but their bond can break, like hurting his feelings because Gaster didn't accept their mercy, or some things he worked hard on and Sans 'ruined it' ( Even sans fixed it after for a second. )
Before the plates
Gaster did not add the Plates screwed into the brothers hands in till some time after being let out of the M-D Solution.
One of the first things that happen to Papyrus after being let out is his arm coming off. He panics. He starts to scream which alerts Gaster to see whats going on, and after a little bit of anger, he got Papyrus's arm back on. Papyrus looks for attention from Gaster, but is rejected.
The next major thing to happen is when Sans and Papyrus interact for the first time. Gaster was clearing out boxes with Sans sitting near by, within ear shoot of Papyrus. They quickly realize that their nearby and start to make ;animal noises' at each other. But soon they begain to babble. bits and piece's that Gaster had said to them. They are sentient after all. This throws Gaster into a short panic before he makes a plan. He will introduce them to each other.