Kira has no problem badmouthing Heart or insulting her, but she actually seems to be a little-less hostile towards her than she is to most. She finds her ideals to be stupid and seems to ignore most of what Heart says.
Both girls seem to have a mutual-disliking for each other and she once addressed Lieselotte as a Snot-Nosed Brat at one point.
She could be considered a "best friend" for Kira, as they can be found together and treat each other with mutual respect. Kira is responsible for helping to make Mei-Fang's skeletal frame, and is the only other person next to Mei Ling that Mei Fang trusts to repair her.
Having attended the same school, Kira seems to consider her a friendly-rival.
During the main story that can be unlocked playing the rest of the game, Weiss is shown to have been with Kira. While it was implied that they had been temporary friends, when the others find her trying to make a large subspace to take over the world, she acts as though she cared nothing for Weiss. However, after the machines she was using are broken and she is knocked unconcious, Weiss claimed that this wasn't how Kira behaved earlier on, and returns her home.