It's not the kind of promise Michiru is eager to make, necessarily, but seeing the relief on Haruka's face— slight as it might be— works as an incentive.

Promising that she won't trade her life for Haruka's doesn't mean she has to let her die, anyway, does it? It just means that she'll have to take a different approach, should the situation ever arise.

She takes a breath, and then:

"I'll give you my word." Her head tips toward Haruka's hand, where her thumb had brushed her neck. "And I'll keep it."

She wants to add in a condition, to make Haruka promise her, too, but she won't. Not when trying to compromise had done more harm than good, earlier.

"We're partners, after all," she adds, softer than before. It feels like they're on thin ice, but saying that out loud helps.