An author, mm-hm. A fictional character—mm, that's a little more complicated. You can't summon something that isn't real, but that doesn't mean there isn't a replica that can be summoned in its place. They're usually weaker, but... It's possible to summon a wraith in place of a usual Heroic Spirit, if you mean to summon an entity of pure fiction. A soul that can take on characteristics akin to the character they're to portray... There's more nuance to it than that, especially when we're talking about folklore where there isn't only a single inspiration behind the tale, but that's the gist of it.

As for becoming a Heroic Spirit at all... Well, I suppose there are two main components: fame and mystery. It's a lot harder for people who were born in the modern era to become Heroic Spirits, even if they were really famous when they died. That's because there hasn't been a lot of time for their legend to grow, and also because there just isn't as much mystery left in the world. It's a lot different now that we have so many cameras around, right? Things can be recorded and analyzed... That's also part of why magecraft has been getting weaker ever since the Age of Gods ended, but that's another topic. Um, to put it simply—the older a legend is, and the more famous it is, the more likely it is that that soul will be summonable as a Heroic Spirit.

Oh, and another thing—a Heroic Spirit has to consent to answer the call of the Holy Grail! That means that, with a few exceptions, the Servant will always have a wish of their own... Some regret from their own life they would like to change, or something they would like to redo, or something like that. But... I guess we already know that's the case with Alice the Servant, don't we? That's what Allen was already told.