Pulling metaphors out of thin air is often difficult and intimidating. To help you creat metaphors that describe someon's personality, I suggest going through the following brainstorm. This has helped many of my students to create meaningful metaphors to use in their poetry. Try the following:
1. On a piece of paper, make a list of personality traits that this person has. For example, funny, strong, selfless, spontaneous, etc. These are words that you would use to describe who they are and what they are like.
2. Once you have a list of their personal traits, try to think of objects, images, colors, smells or tastes or touches that also possess those traits. Next to each trait, list a few possibilities. For example, next to funny, try to think of things in the world that are also funny (a joke, Laffy Taffy wrapper, stand-up comedy, etc.). For spontaneous, what is also spontaneous in the world? (The blooming of flowers in spring, fireworks, Pop Rocks, summer thunderstorms, etc.) Try to do this for the personality traits that you picked; this is pretty tricky, but if you think it through, you'll be able to come up with some.
3. After you have a list of objects in the world that are also similar to your character's personality, pick one that you can use to go in depth with. For example, let's say that for selfless you picked a journal (it's always there for you). Make a list of all the ways that a journal is similar to this person. For example, they listen all of the time. They don't judge. They're always there for you, etc.
4. You are ready to phrase your metaphor. Start by saying "___________ (whoever your character or person is) is a journal (or whatever object you picked from your list), because he/she is giving, always there for you, non-judging and listens all of the time (or whatever reasons you listed). And there you have it! A metaphor with elaborated support and reasoning!
I hope that those thoughts helped; good luck!