Thesis: Many of the most memorable non-player characters in IF are ones the viewpoint character already knew before the game started.
Supporting positive evidence: Michael from Anchorhead; many or perhaps most of the characters in Robb Sherwin’s games; Miss Sierra and Princess Charlotte in Varicella.
Suggested explanation: it’s really hard to get to what is interesting about someone in the first few minutes or hours of acquaintance, so in games where we’re meeting everyone for the first time, a lot of energy has to be wasted on the building of trust and mutual figuring-out.
Also, the viewpoint character can’t make any general observations about the history and personality of other characters, because he hasn’t known them long enough — so the whole tool of direct exposition is off limits. Showing and not telling is good a lot of the time, but telling can be a valuable shortcut when you want to get to the interesting parts of a relationship.