Thelian

Thelianaresth "Thelian" th'Vorothishria was an Andorian thaan who served as an officer in the Federation Starfleet before going on to become one of the most influential Presidents of the United Federation of Planets of the 24th Century.

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. (Titan novel: Taking Wing; 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 the Federation President, used by and containing portraits of every president after Thelian.

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. (novel: Articles of the Federation)

At formal functions, Thelian would often choose to wear his Starfleet dress uniform, as opposed to traditional Andorian garb. (Enter the Wolves; Star Trek: Pendragon

Alternate timelines
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.

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 empassioned plea was ultimately unsuccessful.

Afterwards, Thelian exiled himself from Andoria, shamed by the government's support of Leyton, and instead retired to Betazed. (Star Trek: Pendragon)