Masahiro Sakurai, director of Super Smash Bros. Ultimate, has shed some light on the people that worked on the Lifelight theme song.
Sakurai announced Hideki Sakamoto is the composer of the Lifelight theme song. Sakamoto is known for his work on games in the Yakuza series and Super Smash Bros. for Wii U/Nintendo 3DS.
The Japanese vocalist for Lifelight is Erina Koga, who is only 18 years old yet.
Sadly we do not know the English vocalist yet, or is that also her? If so, wow quite a good transition.