"She wasn't always on friendly terms with him, after they died...in past timelines, she always slayed him when he got in her way. Besides that, it's clear that she would've been willing to sacrifice his life as much as her own in order to destroy every world and the lives that inhabit them. He himself once admitted that Chara, in life...wasn't the greatest person. Even as far back as her death, she's been using him for her own goals, likely many times after that, too, in other, future timelines where genocidal carnage brought her spirit back."
"...But she must've still cared about him to some extent if he she still wanted his help when he had so little power, back when he was trapped in Gaster's lab. Before she finally succumbs to madness and jumps in the lake, will she realize that? Will she feel sympathy for the monster she once called her best friend, once again?"