Seiko Koasa made her international debut on February 19, 2003 at an FIS Race in Rastbüchl, where she achieved a very good placement with a 23rd place. In 2003 and 2004 she took part in other competitions in this competition class and consistently reached the second round. At the Universiade in Seefeld in Tirol , she was able to secure bronze with only six starters.
Almost a month later Koasa made her debut in the Continental Cup in Schönwald in the Black Forest . After attaining 18th place there, she competed four days later in Baiersbronn , where she finished 13th. After a year away from international events, she was not seen again until the 2006/07 season when jumping in Ljubno . After the night jumping in Ramsau on August 19, 2007, she only took part in competitions taking place in Japan. In this context, she made her World Cup debut in jumping in Zaō , where she could not reach the second round. After two Continental Cup events in Lillehammer on September 8 and 9, 2012, she jumped at the European World Cup events in Schönwald and Hinterzarten . She finished her second season in 47th place.