Point density with ModelBuilder

Now I have a lot of files that contain XY coordinates, like the picture, but this picture just contains one day. However, I want to integrate all the files I have.

Secondly, I want to integrate all point files. This is the main problem I have. The "Merge" or "Append" tool did not work for me at all or I did not know how to use it correctly, so I can not integrate all point files.

Actually, this is the model I would like to do but it always pops up the error. It says "You start with the invalid character".