Can a guy be a tsundere

Yes. Anyone can be anything. Some male tsunderes are: England and South Italy from Hetalia, Dylas from RF4, Inuyasha, Yushino and Ritsu from Sekaiichi Hatsukoi, Misaki from Junjou Romantica, Ciel from Kuroshitsuji, Eiji from Love Neco, Ryoumei from Yume Musubi Koi Musubi(rare example of a tsundere 'seme' in yaoi), and Piko(the RL singer). Those are just ones I know of, I am sure there are a ton more. It's a very common personality-type for the "uke" of BL/Yaoi genre too, probably second to the "innocent/naive" type. :3 http://i44.tinypic.com/33wodug.jpg In Binan Koukou Chikyuu Boueibu Love, they've also explicitly called Kinshiro a tsundere, even though he comes off more as more kuudere to me.

I can be a tsundere at times.. Maybe more often than I care to admit. In short.. I agree. Anyone can be any of the personality types.