Steven Universe is a short, stout boy, no taller than an Equestrian Pony. He has light tan skin and puffy brown hair, with his Gemstone located on his stomach much like his mother. his main outfit consists of a pink T-shirt with a yellow star on it, along with blue pants and sandals.
When Steven got corrupted by Sombra, he had a vague resemblence to Lion, and actually grew to about the same size. His face turned into that of a mirror with closed eyes, his head green, and his leonine body fur turning flamingo pink. He had a long tail that ended in a flower, his Gemstone moving to the end of his tail instead of staying on his stomach, and he grew a mane of hot pink petals.
As of My Little Universe II, Steven has aged physically, having grown slightly taller and now having a physical neck. He still keeps his blue jeans and salmon-pink sandals, but his shirt is now a bright blue color with a yellow star in the center, and he also wears a pink varsity jacket with white trims.
Steven is a kind-hearted, compassionate child who tries his best to make sure everyone's happy. Even during his time as a Corrupted Gem, this friendliness is extended towards many people, Gems, Humans, and Ponies alike, and is far more willing to make friends than make enemies. His biggest goal is to keep peace and friendship with everyone around him, and try to live up to the same level as his mother once did.
With his efforts to keep peace, this however makes him do his own thing most of the time: if he believes it's right, then he'll do it, even if the overall statement of his friends say otherwise. This trait showed up a number of times throughout the series, including him releasing Lapis Lazuli despite the other Gems demanding the mirror back, and him willing to turn himself over to White Diamond. This is believed to have come from Rose Quartz.
However, his compassion doesn't show to everyone, as there are a very few exceptions to the rule and there's some Steven simply doesn't like for one reason or another: his dislike towards Kevin is a known example, due to Kevin's behavior towards Stevonnie, and his anger towards White Diamond due to her views on Gemkind as a whole. His most notable lack of compassion was presented throughout Season 8, namely towards his interactions with and against Grogar.
For a brief time, this even altered his outlook as he became more angry, and more willing to fight and even kill, evident when he was training with Jasper. After the end of Season 8, this behavior became far less apparent after Steven reconciled with himself, and worked out his anger by My Little Universe II.
A trait from his Gem side, Steven has the ability to heal others, minor injuries being healed by his spit (to the disgust of a few). His tears are far more prominent in this power, even known to revive the deceased as shown with Lars.
Steven is also shown to be able to summon a bubble as a primary means of protection whenever his shield isn't in use. This bubble is very durable to most attacks, and can even be spiked to be used as a weapon itself. This bubble also allows those inside to breathe underwater.
Ever since the Everstorm, Steven had gained the ability to control the speed of his decent after jumping into the air, being presented as him floating off of the ground. He is still currently working out how to control it.