Sanita (The Runaway Bride)
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Sanita was a guest at Donna Noble's first wedding, in which she disappeared into a cloud of Huon particles. She was also present later on at the wedding reception.
When a Roboform Christmas tree attacked the party, Donna checked in on two children, Michael and Connie, then asked Sanita vaguely to "do something useful" for the other people in shock from the incident. (TV: The Runaway Bride)
Behind the scenes
Since the editing cuts away from Donna while she speaks the line, it's unclear if she's speaking to the other woman who's seen checking in on the children, or if she's walked away from them already by this point.