[Kinny Riddle]

Like I said before in a previous episode thread, it seems author Ohba Tsugumi had deliberately left enough hints for the reader to guess who it is all along.

Spoiler for Yotsuba-Kira hints:

In every Yotsuba board meeting, it's always Higuchi that actively suggests having whoever they want dead killed. The other board members simply went along with his suggestions and even made improvements to it.

One example is after Matsuda got busted, while two of the board members go to interrogate Matsuda and the other board members discuss what to do with him, it was Higuchi who first suggested having him killed.

In the same episode later on, one of the board members flips out and wants out of this Killing Meeting. Again, Higuchi was the first to declare his death sentence by saying he will die the next day. From that moment on, he was a dead man walking.