What can or cannot have an item is governed by Wikidata's notability policy:

1. It contains at least one valid sitelink to a page on Wikipedia, Wikivoyage, Wikisource, Wikiquote, Wikinews, Wikibooks, Wikidata, Wikispecies, Wikiversity, or Wikimedia Commons.
2. List itemIt refers to an instance of a clearly identifiable conceptual or material entity. The entity must be notable, in the sense that it can be described using serious and publicly available references.
3. List itemIt fulfills a structural need, for example: it is needed to make statements made in other items more useful.

so there can be items without Wikipedia articles if they correspond to some non-Wikipedia wiki page (e.g. a Wikisource source), they help extend the graph of Wikipedia items (e.g. relatives of a famous person who can be linked via child etc. relationships but aren't notable enough to have their own Wikipedia articles), or if those items refer to things or concepts which are discussed in serious sources (e.g. peoplevtend to mass import things like authors from scientific databases).

Unicode characters presumably fall under the second point, the Unicode standard being a serious reference.

What can or cannot have an item is governed by Wikidata's notability policy:

1. It contains at least one valid sitelink to a page on Wikipedia, Wikivoyage, Wikisource, Wikiquote, Wikinews, Wikibooks, Wikidata, Wikispecies, Wikiversity, or Wikimedia Commons.
2. It refers to an instance of a clearly identifiable conceptual or material entity. The entity must be notable, in the sense that it can be described using serious and publicly available references.
3. It fulfills a structural need, for example: it is needed to make statements made in other items more useful.

So there can be items without Wikipedia articles if they correspond to some non-Wikipedia wiki page (e.g. a Wikisource source), they help extend the graph of Wikipedia items (e.g. relatives of a famous person who can be linked via child etc. relationships but aren't notable enough to have their own Wikipedia articles), or if those items refer to things or concepts which are discussed in serious sources (e.g. people tend to mass import things like authors from scientific databases).

Unicode characters presumably fall under the second point, the Unicode standard being a serious reference.