Virgil Ivan Grissom Pilot
USAF combat veteran Gus Grissom was a fighter test pilot immediately prior to joining the NASA astronaut program. Following his Mercury flight he would be involved in the Gemini Program from the start, helping to design the flight control systems and commanding the first crewed Gemini mission. He continued to work on the program until 1966 when he was selected to command the ill-fated Apollo 1 mission.
Gus Grissom in words
See this quote by Gus Grissom in the transcript
I'm trying the yaw maneuver and I'm on the window. It's such a fascinating view out the window you just can't help but look out that way.
Alan Barlett Shepard, Jr. CAPCOM
Having become the first American in space in Freedom 7 earlier that year, US Navy pilot Alan Shepard served as Cape CAPCOM for the two subsequent Mercury flights, Gus Grissom's in July 1961 and John Glenn's first American in orbit in February 1962. Plagued with inner-ear problems which grounded him for most of the rest of the decade, Shepard was restored to flight status to become the only Mercury 7 astronaut to walk on the moon in 1971.
Donald Kent Slayton Blockhouse Comm
Air Force test pilot Donald “Deke” Slayton had also been selected as one of the “Mercury 7” astronauts in 1959. However, a heart murmur would lead to him being medically disqualified from flight status before Mercury-Atlas 7. He subsequently became the Director of Flight Crew Operations, and was known throughout NASA as “The Chief Astronaut”. In 1972, his flight status was restored, and in 1975 he made his only space flight on the Apollo-Soyuz Test Project.