Lord Tommen Costayne is the head of House Costayne and Lord of Three Towers.
A Game of ThronesEdit
Lord Tommen presumably follows his liege lord, House Tyrell, in declaring for King Renly Baratheon during the War of the Five Kings.
A Clash of KingsEdit
Tommen presumably follows Lord Mace Tyrell into declaring for King Joffrey I Baratheon after the death of Renly.