Ever17 Biology Clue

Attn: Doppel, anyone else interested in Ever17 and has already completed Tsugumi's arc

All you need to know to appreciate this is that Tsugumi's hamster was a Dzungarian, and "Dzungarian hamster" is an older way of referring to the Campbell's dwarf hamster and its closely-related relative the white Russian something-or-other:

"Campbell's dwarf hamsters have an average lifespan in captivity of either 18-24 or 18-36 months - sources vary."

Hello, major plot-hint. If you knew this factoid while playing Tsugumi's arc for the very first time, the scene where she first introduces Chami to you (and mentions how long she's had him) would pretty much be a major, major red flag that something's not right. :)