By definition, the MIKE linebacker is the middle defender in the trio of base off-the-ball linebackers commonly seen in a even front (4-3 base), or the shifted strong side backer in an odd front (3-4 base). This player is often known as the quarterback of the defense, and is usually responsible for setting the unit based on the offensive formation, personnel grouping and down and distance.
In the past, the MIKE linebacker was known as the downhill thumper, who was excellent in run support, but was not the best when in came to defending the pass. You would often see guys hover in the 245-260 range, and their common pass responsibility was high-hole deep sit, intermediate hook-to-curl, or spy the quarterback in case he took off running.