Annie is Doctor Strong's much younger wife. She is the object of a ton of gossip and suspicion – including from Mr. Wickfield – because she appears to use her husband to arrange a job for old childhood sweetheart and cousin, Jack Maldon. In fact, everyone thinks she's having an affair with Jack. However, it turns out that appearances aren't to be trusted: Annie's mother, Mrs. Markleham, is behind these arrangements, and Annie doesn't feel much for her cousin; instead, she is completely loyal and devoted to her husband. For more on the weird politics of this whole thing with Doctor Strong, check out our thoughts on Doctor Strong.
Annie has one other notable part in the novel. When she is telling Doctor Strong her feelings for him, she gives him a reason for why she has never been seriously attracted to Jack Maldon. Her reason is that, "There can be no disparity in marriage like unsuitability of mind and purpose" (45.69). Annie means that Jack Maldon is too light and flighty for her; she wants a steady fellow like Doctor Strong in her life.
But these words haunt David, because they finally bring home his own troubles with Dora: David realizes that, even though they both love each other, he and Dora are definitely unsuitable in mind and purpose. David's just a lot more serious than Dora is, which proves damaging to both of them emotionally. Annie's words have the unintended effect of convincing David – even while Dora is still alive! – that he should never have married her.