[@KenjuGuy] The Witch had to endure centuries of torment and trauma, so it's a miracle she's got any kindness left in her heart. As it stands, her moral compass is a tad warped, such that she has no problem killing humans to save humanity. However, she's still fundamentally a "lawful good" sort of person at heart, so she would find it useful to have someone keeping her on the straight and narrow.

Angels and demons are types of Higher Spirits, which, unlike Mortal Spirits, are capable of existing independently from a body. Mortal spirits will disperse without a body, so angels are typically tasked with dragging them to the afterlife when their spirits separate from their bodies (i.e. die). Demons can't really stand in the way of that, but they are absolutely capable of speeding along the dying process. A guardian angel's purpose, then, is to prevent death by external means for as long as possible.

Obviously, the Witch doesn't need that, since she has ascended to the level of a Higher Spirit (i.e. roughly equivalent in rank to an angel). However, as we've both noted, she could still use someone to keep her on the straight and narrow. Additionally, angels possess miraculous powers, and if the Witch were to contract with one, she could acquire magical abilities, with the restriction that she can only use them when the angel deems it appropriate.

In other words, if she contracted with your angel character, he could control her moral judgment through the exercise of magic power.