Outcast Ancient, forced to remain on the dead planet Velona following a terrible tragedy. Orlin came upon the world of Velona long ago, and took pity on the people's struggle against the Goa'uld. Breaking the rules of the Ancients, Orlin gave the people Ancient technology, particularly weapon specifications to Velona scientists (it is doubtful he revealed himself directly), from which they constructed an advanced Ancient space weapon.
It was not long before the Goa'uld presence was destroyed, and the people celebrated. Unfortunately, this new-found technology birthed ideas of power and corruption, and they soon planned to exploit their enemies on other planets, using the weapon to conquer those worlds. The Ancients saw this possibility and decimated the entire civilization, leaving their cities in rubble and Velona's people extinct. Orlin, for his disobedience, was banished for eternity amongst the wreckage he had instigated, to walk among the dead for all time.
Orlin's fate changed with the arrival of SG-1. He was immediately taken by Samantha Carter, and tried to merge with her thoughts, but the unprepared scientist blacked out. Be it secretly or known, Orlin was allowed to leave the planet with SG-1, concealed in an invisible form, to walk among the humans, particularly Carter. He returned with her to her home and remained inside her residence, watching her while she slept. The next morning he showed himself to her, and almost immediately told her he had fallen in love. He proved he had come through the Stargate by walking through a table.
Carter left the house and sent in a search team, but Orlin was not to be found. He only returned after Carter believed all of the surveillance devices were gone, which was not the truth. Orlin revealed himself to be an ascended being. Carter told him of Oma Desala, but he had no knowledge of her because she had been an outcast.
For weeks Orlin seeped into Carter's confidence. They took walks together and discussed the future and his presence. Orlin eventually gave up his powers as an ascended being altogether, becoming fully human again, and planned to spend the rest of his life with her in that form. But the pentagon had been observing him and his very real presence with Carter, observing acquisition requests being sent to Carter's home. While she was on-base, he ordered parts and equipment from the Internet and developed a miniature Stargate in her basement.
When the N.I.D. finally moved in on the house, Orlin activated the Stargate and traveled back to Velona, Carter following. He had discovered the humans were planning on studying the Ancient weapon technology, and refused to accept that the same fate fall upon Earth. After being shot by a member of SG-16, the Ancients, in their wisdom, found Orlin to be redeemed. They re-ascended him as a reward for his great attempts to sacrifice himself for the Tau'ri. After he transformed, he took the core of the naquadah reactor powering the space gun, which could not be stopped from activating, into the atmosphere, where it detonated. Immediately he (or the Ancients) destroyed the Ancient space gun so that SG-16 could never replicate the technology.
Four years later Orlin descended upon Earth in human form, this time taking on the physical appearance of a young teenage boy. He was aware of the Ori threat in the Milky Way Galaxy and had returned to lend his expertise in developing an antidote for the vicious Prior plague. Because of the vast Ancient knowledge in his mind his return as a boy allowed him to retain more of this information, but the sheer amount of information began to inflict damage on his brain almost immediately.
While Orlin tirelessly worked to develop a cure his brain became more and more damaged to the point where even simple tasks were nearly impossible. Eventually he reached a point where he could not proceed further, and he created instructions to aid Sam in finishing the cure. With the help of the Jaffa Gerak, an antidote was completed, but all of the Ancient knowledge was emptied from his mind. Orlin, now just a boy, was institutionalized.
Ascension - The Ancient Orlin falls in love with Samantha Carter and determines he wishes to spend the rest of his life with her, but a call back to Velona forces him to choose the entire people of Earth over his love.
The Fourth Horseman, Part 1 - Orlin returns to Earth to help find a cure for an Ori plague ... as a young boy.
The Fourth Horseman, Part 2 - Orlin continues his work on the Prior plague, giving all of his memories in its making. He is successful, but not before all of his knowledge is lost and he is truly reverted to a child mentality.