Seeing a person in a dream who has passed away in real life can represent that person, your memory of them, a characteristic of theirs (optimism, creativity, etc.), or something else you associate with them.
The person might be on your mind because you’ve been thinking about them, you’re missing them, a significant date such as a birthday is approaching, you’re curious how they’re doing or what they’d think about something in your life (or some other reason).
See Also: Person You Know, Dead Body, Death of a Loved One, Dying
[1]