The Ultimate Force (called the System Force in the Dark Horse releases) is the program in Yggdrasil that makes sure a contract works out. If anything threatens to separate Keiichi and Belldandy, the Ultimate Force will remove that threat. The stronger the threat, the stronger the Ultimate Force.

Later, the Ultimate Force doesn't matter as much, as Keiichi and Belldandy are a couple out of their own volition, and don't need the Ultimate Force to reinforce it.

From MangaWiki, a Wikia wiki.