Is there a way to claim someone as a dependent who is not a relative or partner but whom you have been financially supporting?

You can contact the CRA to double check, but most credits that you can get from having dependants require them to be a relative.

If you are in Canada or the United States, you can call the Canada Revenue Agency (CRA) at 1-800-959-8281. If you are elsewhere you can call 613-940-8495. This page has more ways to contact the CRA: https://www.canada.ca/en/revenue-agency/corporate/contact-information/telephone-numbers.html