He was elected by the boyars to replace the ineffective Mitso Asen as emperor of Bulgaria in 1257, when he married Irene of Nicaea, a daughter of emperor Theodore II Doukas Laskaris by Elena of Bulgaria, the daughter of Ivan Asen II of Bulgaria.
He was elected by the boyars to replace the ineffective Mitso Asen as emperor of Bulgaria in 1257. To reinforce his position as legitimate ruler, Constantine adopted the name "Asen" and married Irene of Nicaea, a daughter of emperor Theodore II Doukas Laskaris by Elena of Bulgaria, the daughter of Ivan Asen II of Bulgaria.