Well, I have 4 MCs (brothers), sooo...
Daitaro would be really happy since he has a fire constitution (can turn into fire) and also radiates heat (like human body heat but more like the feeling of a radiator when sitting close to him) Before the fire thing though, he would probably like to think on ways his brothers can manage to survive the cold and be super attentive to them wearing cozy and appropriate clothing for it.
He tolerates the cold more than his brothers so he'd be more into wearing comfortable clothing and buying heat pads for his brothers to use
Daiki, his twin brother would try to have him always nearby as to get the benefits of his radiator like effect. (Though, when not together, he would probably be covering himself with blankets and big jackets while cursing the cold)
Masaru would, besides sticking to Daitaro as often as possible, try and exercise whenever possible in an attempt to fight the cold. Though he would probably use the cold weather excuse to drink lots of hot chocolate and try to make his brothers join him for a campfire.
Whereas, Mitsuru, would shut himself in his room and become some sort of blanket caterpillar while refusing to leave the house because 'it's cold outside'. He later has ice powers so it doesn't really bother him as much, kind of Daitaro's situation but ice.