Hikaru sucking on Haato's finger. You have to make Hikaru a submissive. As for getting him to suck on Haato's finger... I usually do the action a few times and he eventually gets it (not sure if that's how it works, but it did for me)
You have to use the Mouth Icon on his penis. (It doesn't work at the first time. You have to do it several dates.) At least I needed 8 dates until I get the chibi. And your lover have to be the dominant part. (I hope my english is good enough > < I am not a native speaker)
