Tengu is horrible character and people that use him should just pick a decent character.
In DOA4 and DOAD, yes he was. Sometimes people just click with certain characters though. When this game first came out you were absolutely convinced that Helena was the worst character in the game, but you still played her instead of just picking a decent character.
Besides, it's not like he always has to suck just because he did in his last two games. He was great in DOA2U. I've discussed various times with other players how they could make him better and it really isn't hard. For the most part, he just needs more mixups, and faster frame data. A 15 frame jab is absolutely ridiculous and uncalled for. Shave it down to 11 frames like the rest of the slow jabbers.
Leon I just don't want to learn how to play against since only 3 people will use him.
That was the case in DOA4 because it was DOA4 and he wasn't a game dominating ninja. In DOAD, he just sucked. I'd easily imagine that people would pick him up if he were good, which again, is totally possible. Just give him some buffs. Besides, the simple fact that he's a "new" character will cause plenty of people to try him out and if he was buffed enough, they might stick with him.
Just try to put yourself in our shoes for a minute. Imagine that they cut Helena and Lisa (who I think you submain) from DOA6. Naturally, you're not going to be very pleased about that. Sure, you'll pick someone else because you have to, but you'd always want your real mains to come back and once they did you'd probably switch back to them.