 If you liked
Ai-Ren
|
 ...then you might like
Sankarea
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Both Ai-ren and Sankarea are about artificially created girlfriends with an uncertain time span. Both are really god and worth reading. Beware though, even if Sankarea actually contain more fan service than Ai-ren (which isn't fan service centered at all), Ai-ren is still "seinen" and darker.