This guy is maybe really nice to the girl and his 'fanny' persona may be a defence mechanism for low self esteem. He might also have a big, long and fat cock that he uses to pelt her rotten.
There are ways of letting this girl know that you like her without looking like you are trying to cut this guys grass. Put a wee seed of thought in her mind and if she comes to you, she comes to you. If she doesn't, f**k her! Not literally of course because that would likely land you in the jail for rape.