Warning: This section contains spoilers for all six books in The Mediator series!
Hector "Jesse" de Silva was born to a wealthy pair of Spanish ranchers in California in 1830. He was the only male child and had 5 sisters ranging in age from 6 to 16 at the time of his death (1850). He secretly wished to become a doctor but couldn't because of his responsibilities to the ranch.
In the year 1850, Jesse was engaged to marry his cousin, Maria de Silva, but discovered that she was cheating on him with a slave trader named Felix Diego. When Jesse learned of her affair, he decided to call off the wedding; while on his way to do so, he was murdered by Diego in his sleep while staying at a boarding house in Carmel.
Jesse's spirit still haunts the room in which he was killed when Suze moves in, about 150 years later. Despite a rocky start the two become friends and eventually develop romantic feelings for each other - feelings they try to deny due to the difficulties of human-ghost relationships. Jesse frequently refers to Suze as "querida," which means "dear one" or "beloved" in Spanish.
Jesse can most often be seen wearing a contemplative frown, but he smiles easily and is generally good humored. He has a thin scar through his right eyebrow, which came from a dog bite. Jesse has very strong morals; he will not go further than kissing or hugging Suze, and is very set in his ways concerning moral issues. Jesse can become forceful and stubbornly protective when Suze's safety is in question, leading to arguments between them. He will also act in Suze's best interests instead of his own, leading him to move out of her room when they begin dating.
He displays a great interest in history and human nature and can often be found reading a pilfered textbook on the window seat of the room he and Suze share.
As a ghost, Jesse is stronger and faster than most humans, and due to his non-corporeal state can walk through solid objects (including humans who do not notice) and is immune to most physical damage. However, he can be touched by mediators and other ghosts, as well as making himself known to normal humans if he needs to. He can also appear or disappear at will and cross great distances incredibly quickly.
As with all ghosts, he has psychic abilities - which can take several forms, though they seem to be limited to fairly powerful telekenisis - which he loses control over when he is angry or upset, causing objects in the room to shake as if in an earthquake.
At the end of the sixth book, Jesse becomes human again when Suze inadvertently brings his body forward in time, allowing his spirit to re-enter it. After he becomes human again, Jesse becomes a mediator, retaining his human strength and speed. Whether he is a shifter like Suze and Paul is not revealed, as is whether he keeps his psychic abilities.
Information from Wikipedia