"He was innovative, true to his art and continually relevant even though he never cared about being relevant," Cooper wrote. "He was always creating and redefining hard rock and the role of bass within it. Offstage, he was a gem. I can't think of anyone who didn't adore Lemmy. He was such an original character in rock, and I will truly miss seeing him out on the road."
Cooper extended his condolences to Kilmister's family, bandmates and "rock fans everywhere," adding to the growing number of tributes pouring in from across the musical spectrum.
Elsewhere, Metallica paid their respects to Kilmister with a note on Facebook that read, "Lemmy, you are one of the primary reasons this band exists. We are forever grateful for all of your inspiration. Rest In Peace." The metal outfit also shared a live clip of them performing Motörhead's "Too Late, Too Late" alongside Kilmister.
"I'm so glad we were able to get some quality time with him recently," Metallica bassist Robert Trujillo tells Rolling Stone. "He was really the coolest, the smartest, and funniest gent I've ever met. R.I.P Lemmy! Killed by Death - Immortalized by Rock and Roll."
Motörhead and Metallica have a long shared history, with the former picking up their first Grammy for Best Metal Performance in 2005 for their cover of Metallica's "Whiplash." Metallica also covered Motörhead's "Overkill" for their 1998 covers album, Garage Inc..
Rage Against the Machine guitarist Tom Morello also chimed in on Twitter, calling Kilmister "a true rock icon, singular in voice and spirit. Metal flags fly at half mast tonight as we salute the incomparable Ace Of Spades." Red Hot Chili Peppers bassist Flea wrote, "Lemmy got let in on the big secret. One of the greatest rockers of all time. Amazingly unique incredible bass player. My hero."
"Lemmy was a rock and roll legend, icon, superstar, ALL OF THE ABOVE!!!" Slayer guitarist Kerry King tells Rolling Stone. "We will never see another like him, and he left a gigantic hole in the music we all love. Sad, sad day. Cheers to my friends in Motörhead, my thoughts are with you. Lemmy- reign in peace."
WWE Raw also paid tribute to Kilmister during their broadcast Monday. Amongst his myriad non-rock related interests — including slot machines and Nazi memorabilia — Kilmister was an avid wrestling fan and maintained close ties to the sport, with Motörhead performing at several WrestleManias and star Triple H using the band's "The Game" as his entrance music.