As mentioned before, he does not do much other than just walking around. Although he can be killed with the axe effect, if you go outside and then go back into the library, he'll be alive and well again.
However; there is an event that can be triggered where the library may appear to be warped and the boy has become a strange creature. When you're in the Night Woods, you may hear a weird noise. When you hear it, enter to the library and the event will trigger.
The event ends when Chie goes out of the library and chances are that when you go back in, the boy has gone back to normal.
There's a secret counter to trigger the event: while you're in the Night Woods, a random number from 0 to 3 adds every second to the counter. When the counter reaches 50, the noise will sound and you have around 3 seconds to enter the library to make the event happen. If missed, the counter resets and is ready for start anew (By the average 1.5 counts adding, the noise could expect to be heard around every 33 seconds.)