He's not really flirty by nature so there isn't multiple girls he flirts with. Its usually just one of his female friends who he used to hook up with, they seem like really good friends and sometimes he'll be overtly sexual towards, his behaviour would make you think he's single. He's also very touchy-feel when he's with her.
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he's probably just comfortable like that around her because of the history but if he's with you then he shouldn't be doing that shit with another girl.0