 If you liked
Hidan no Aria
|
 ...then you might like
Sakura-sou no Pet na Kanojo
|
Both male protagonists have more than one girl going for them in the series (although I wouldn't consider either of them to be "serious" harem by any means), and at least one of those girls in both series shows their love for the protagonist in creepy, overbearing ways and demonstrates extraordinary skill in a certain area - although Sakurasou probably pulls this off without being as cliché or unrealistic.