Thelianaresth th'Vorothishria, more commonly called "Thelian", was an Andorian thaan who served as an officer in the Federation Starfleet before going on to become one of the most influential Federation presidents of the 24th century. (Star Trek novel: Articles of the Federation; TOS comic: Enter the Wolves)
Thelian was the Federation president when first contact was made with the Cardassian Union. Thelian attended a historic meeting with the Legarans at Vulcan in 2327, to which the Cardassians had been invited as observers. However, Cardassian efforts to sabotage the talks and assassinate the Legaran delegation, though disavowed by the Cardassian government, served to damage diplomatic processes, leading eventually to the Cardassian Wars. (TOS comic: Enter the Wolves)
While in office, Thelian had a château built in the Loire Valley to serve as his home. Château Thelian, as it came to be known, became the official residence of Federation presidents, used by and containing portraits of every president after Thelian. (Star Trek novel: Articles of the Federation)
By 2380, Thelian was the eldest surviving former president of the United Federation of Planets, and upon the death of Jaresh-Inyo of Grazer that year, one of only two living former presidents. The elderly Thelian's poor health prevented him from attending Jaresh-Inyo's state funeral on Mars. (Star Trek novel: Articles of the Federation)
In the Pendragon timeline, President Thelian spoke out against the declaration of martial law on Earth in 2372, and was a vocal opponent of Admiral James Leyton's military coup. When the Andorian government gave their support to Leyton, Thelian was placed under house arrest and prevented from making public statements. (Star Trek: Pendragon: "Sword of Damocles, Parts I & II", "Patriot's Insurrection")
After the end of the Federation Civil War, Thelian attended the state funeral of Jaresh-Inyo on Mars. He then traveled to Earth and addressed the Federation Council, urging them not to isolate the Federation from the rest of the galaxy. Thelian's impassioned plea was ultimately unsuccessful.
Thelian may have been the Andorian representative on the Federation Council in Star Trek IV: The Voyage Home.