From the Super Mario Wiki
Stephen Mortimer is a senior game designer currently working at Riot Games in Los Angeles. During his higher education studies at the DigiPen Institute of Technology, Mortimer started working as an intern at the Nintendo Software Technology Corporation, where he continued to work for eleven years. In this time, he was promoted to lead game designer and eventually game director. His mission at the company involved designing game mechanics and levels for various Nintendo puzzle games, including Mario vs. Donkey Kong titles. For the latter, he specifically designed or assisted design of the games' integrated level editors, as well as mechanics such as Pink Blocks and boss battle behaviors. He also created a large number of levels for each game, both shipped along with it or released subsequently as online stages. As he gained more prominent roles with each game in development, Mortimer eventually took over director duties for all games starting with Mario and Donkey Kong: Minis on the Move, becoming the youngest director for a title shipped by Nintendo.
Mortimer is featured in a YouTube video posted on Nintendo's channel, in which he offers tips for level creation in Mario vs. Donkey Kong: Tipping Stars. He was known as "Mort" on Miiverse, where he consistently updated the Mario vs. Donkey Kong: Tipping Stars community with posts regarding new official online levels, which he also produced. After leaving Nintendo, he opened a YouTube channel, "First 30 of Game Design", where he played and discussed the game design of various video games, including Paper Mario: Color Splash and Super Mario Run, before the channel went inactive.