What scene am I missing? According to some guides it's in Sakura extra event but no guide tells me what it is. I have seen people post pictures of the sprite and none of the girls have costumes that look like that.
If you thoroughly played through the Salacious Sakura event, you should have found a charm in there which unlocks Akane's bunny outfit. After you have her equip it, you need to visit her in her bedroom during the Morning or Midday. Hopefully you can figure out the rest from there.