It's Peter!!! The character is seen for the first time in ages on the episode "Subject 9."
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A frightened Olivia is woken up inside her apartment here. It's a shot from "Subject 9."
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Fringe Season 4 Episode 4 Quotes

You were always the strongest. The others recognized that. You were always the favorite.

Walter

So you're saying that your equipment somehow recorded the event before it even occurred. How is that even possible?

Astrid