You can't say if it's needless if you haven't seen it yet, Ergo. That's just your opinion on DLC in general, and it's influencing your judgement and perceptions. SE could think of quality ways of expanding her story that hadn't occurred to them in the process of making Part 1 itself but which what will be shown to us in Part 1 may leave openings for. Part 1 is complete, but that doesn't mean that additional content can't be created, content that wasn't thought of originally but might be considered at a later time if it can enhance the already existing world and its characters. Sometimes companies don't think of everything at once when creating the base game, and that's what such DLC can help with. If SE has shown anything about FFVIIR, it's that it's not been merely about the money. And I don't think any such DLC would come from that motivation, either. It would be to give us more content, story, and gameplay than they originally thought of but which could add to FFVII and its experience. And give us something to tide us over until Part 2.
And Ry, I can see the possibility of such DLC's for Biggs, Wedge, and Reno, for example, because like Jessie they all have some combat ability and thus, the foundation for playability. The others though, not so much. Aerith is already a PC so she doesn't need it - she gets enough attention already, and she'll have multiple games in the remake series in which to shine. The same with Rufus, at least for now - he has some combat ability, but I think a DLC for him might work better later in the series. Plus he'll get attention anyway as one of the primary villians of the series. The point of DLC like this should be to give more attention to characters who aren't PC's or main villians but who still play a significant role in the story and have some combat ability to make playability feasible.
You never know, Ody. SE's given her more screen presence than either of the other two. And there's no reason they couldn't still be in her standalone DLC - they are her friends, after all. It would be odd if they weren't there at some point.