I have a lot of what I consider OTPs from different series, but I have to say that these two are definitely mine from Shiki. Although nothing really happens between them in Shiki, there are a lot of implications that there may be something a bit more beyond friendship. This is especially true with how Tohru never gives up on trying to get close to Natsuno (even using his first name all the time when Natsuno hates that), no matter how much he tries to push him away, and even doesn't mind that much that Natsuno tends to take his bed when he sleeps over.
There is the part where he claims he wants Natsuno to go with him to ask a woman out, although it never happens, but there are theories that make sense that he may have been looking for an excuse to ask Natsuno instead too (just theories, no one will ever really know). Natsuno also seems incredibly sad to see Tohru's staked body as well, and when he first learns that Tohru is a vampire, he states that all of his good memories of Sotoba have Tohru in them.
Also, for me, his mourning of Natsuno after he thinks he dies is very telling to how deep his feelings are for me. While it can be taken as deep friendship, it could also be said that he may have loved him as more then that.
No matter what though, I still consider them an OTP for me, and I hope everybody enjoys the wallie. I worked hard to get it looking just right, and thought that the pinks and sakura suited them well