When taking a look at a few of Kaaatie's/A3DGhost's work, we can see some implications of sorts that may confirm that Claire was once friends with Oliver/Katie and Δlice in the past.

There's only two examples for Oliver's/Katie's case, both of which taking place seemingly in Claire's childhood.

The first example is that Oliver/Katie is teaching Claire and Engel about Archeology, to which Claire and Engel seem to be invested in.

The second example being Oliver/Katie holding hands with Claire in the middle of the forest, with the original caption being "I miss you".

Now for Δlice's case? In this artwork, we can see Claire, Biscuit, Daniel and Anova hanging out outside nearby a barn, just playing around and stuff. If you look closely, you can see Δlice standing next to Anova, albeit slightly faded out.

From those 3 artwork, they may imply that Claire (As well as possibly Engel, Biscuit, Daniel and Anova) used to be friends with both Oliver/Katie and Alice.

Now, if they were friends in the past, why are they all of a sudden enemies? Especially when both Oliver/Katie and Δlice are responsible for Claire's death in the Basics in Behavior animation (Although Basics in Behavior is considered non-canon)?

Well, that's up for yall to speculate cuz I have no fucking clue lmaoo.