This is more for the newer people, but I myself was a bit fuzzy on the details after a year since the last valentine event.

When your doing any of the quests that are on the Dance Club section of the event, you can head into one of the bathroom stalls on the top left area and enter an underground section. You'll face four groups of succubi and at the end you get:

1000 character shardium
25 valentine token
speed succubus ring

You can come back and fight the succubi again, which is an easy way to get the achievement of killing them, but you only get the items one time.

Not sure anyone will actually read this as the forums have been dead for some time. Aw well, maybe it will reach those who need it. I'm gonna go take a bath in some soup now.