Sopo, sometimes stylized as SoPo, is a term used to refer to playing as only the lead Ice Climber, often due to the partner being KO'd. The term comes from a combination of "solo" and "Popo" (the lead of the default costume). It is sometimes known as Sona or SoNa for people who use Nana as the leader, though many will continue to use "Sopo" regardless of the costume selected.
Sopo is more of a legitimate character than the similar Nolimar, as Popo's regular moveset remains mostly effective when Nana is absent. That said, he does lose significant effectiveness, such as not being able to use a meteor smash or most chaingrabs, and is unlikely to recover from moderately strong blows due to not having a fully-functional recovery.
There have been some tournaments that allow a single Ice Climber to be used in low tier events, though this was more commonly found in Melee as it was significantly easier to suicide Nana by just Belaying too low. Some Ice Climbers mains are famous for their use of Sopo, such as Chu Dat in Melee and Vinnie in Brawl. There also used to be a combo video on YouTube which only consisted of Chu Dat's moments with Sopo. Some of the most famous Sopo matches include a match of 9B's Sopo vs. ADHD's Diddy Kong during Apex 2013, where 9B took two stocks off of ADHD during game 3, and a match of Fly Amanita's Sopo vs. Silent Wolf's Fox during Kings of Cali 3, where Fly Amanita made a comeback using Sopo during game 5.