In "The Dissociation of Haruhi Suzumiya" and the following book, can the character Yasumi Watahashi be considered a slider? (I'm not naming her in the title because she's not in the anime).
Since she essentially travelled between the two timelines and her purpose was, if I remember correctly, to reunite them. I read these books a few years ago (in a fan translation) and while I was pretty convinced she was the long-awaited "slider," most of the fanbase seemed to think she wasn't. I couldn't find any good reason why she should or shouldn't be or, in fact, any canon or otherwise generally accepted definition of what a slider is - I've always assumed it was someone who travelled between alternate timelines.
I'm rereading the beginning of Dissociation right now and Kyon reminds us that a slider has yet to make their appearance, reminding us that we should naturally expect one and, IMO, maybe foreshadowing the end of this arc and implying that Yasumi, even though we learn later that she is not a real individual, is "the" slider we've been looking for.
I'm aware that the series has been left unfinished and we will likely never know if another slider was planned.