The developers have even admitted to it when discussing them in interviews.
"Because a big part of the fan base is guys, we always put in a nice sexy character, hence Gloria. In the past we’ve used Trish, who is a recurring character, and also Lady. Lady appeared in 3, and now she’s about ten years older than she was in that one, but she’s grown up very nicely, as you’ll see."
----Hiroyuki Kobayashi, Devil May Cry 4 Interview
Their literal excuse for including Trish and Lady was for the sake of fan-service----not continuity, not narrative fulfillment, fan-service.
I don't know what you expect her to do. Last time I checked, the Order of the Sword didn't equip their singers with Devil Arms or combat training. She's a
singer. Just be thankful she isn't dressed as scandolously as the other females in the DMC universe.
Her presence in the game actually makes sense. While Lady and Trish are thrown in for eye candy, Kyrie actually plays a prominent role in the plot. Her relationship with Nero makes more sense, given they're childhood friends...that's more incentive for their romance than the utter lack of chemistry between Dante and the females in his story arcs.
I'm also happy you conveniently skipped over me mentioning how she saved that child. You know---having no combat prowess or means of escape whatsoever, and still proceeding to use herself as human shield knowing full well she might be cut to pieces---one might consider that pretty selfless, and fairly useful.