Positive:
* Hopeful
* Cheerful
* Upbeat
* Enthusiastic
* Positive
* Sunny
* Bright
* Joyful
* Confident
* Sanguine
* Buoyant
* Radiant
Other:
* Glass-half-full (idiomatic)
* Seeing the good in everything
* Positive-minded
* Unwavering
* Resilient
* Determined
* Motivated
* Inspiring
* Encouraging
* Trusting
* Faithful
You can also use words that describe their actions and behaviors, like:
* Motivating
* Supportive
* Encouraging
* Inspiring
* Uplifting
The best adjective to use will depend on the specific context and what you want to emphasize about the optimist.