What encryption shoud I use for single char encryption? I'm very new to encryption and I wanted to know if it is secure or just possible to use encryption on single chars. I did some research, but nothing to clear and i'm a bit confused.
The modes I think it is possible to do it with are one-time pad, RSA and AES in counter mode. 

These are probably wrong ,but I have no idea why. So please explain me why these can't be used or maybe can be used.

EDIT: since I'm building a keylogger I could just buffer up and encrypt the whole buffer, encrypting a single char is not necessary.
There is no minimal message size. Even an empty message can be securely encrypted, i.e. the ciphertext is indistinguishable from a different encrypted message, as long as the encryption key remains secret.

Can you describe exactly what are want to achieve?

Where will the secret key come from?

Even though minor optimizations are possible with very short messages (in particular, short tags may become acceptable), you should use traditional constructions. ChaCha20-Poly1305 and AES-GCM, in particular, don't require padding, and work with short messages just as well as with long messages.

Edit: since your use case is a key logger, you probably don't need to encrypt individual characters. Buffer them up to a reasonable size (maybe 1KB), and encrypt the buffer once it's full or when the application terminates.