Since Lippies V.4, Knuckles has had a couple characters made to "replace" him. The most notable ones of today are Nyro the Leopard and Thatcher, both which are in the current Lippies Trilogy.
Nyro the LeopardNyro the Leopard, full name Nyro Fletcher (formally known as "Kniro the Amur Leopard"), is a fictional, anthropomorphic red Amur leopard. He was first created to replace Knuckles for Lippies V.5. Nyro has since became the iconic character used by the creator, who often goes by Nyro's full name (instead of using their real name). Though Nyro was originally supposed to have the same personality as Knuckles from Lippies V.3, his personality is actually quite different now. He is more out-going and optimistic. Nyro is also a cyborg, which unintentionally resembles the stupid future Knuckles from the dumb Ken Penders comics.
Nyro's best friend is Tito, a friendly mink with an affinity for math. While they were first only considered friends with a backstory similar to Knuckles and Tails' in the old Lippies stories, they are now considered brothers (Nyro being adopted). Nyro also had a good childhood as opposed to his old backstory and Knuckles' backstory, since he was raised by Tito's parents. Though unfortunately, he was stolen from his real parents by Dr. Lee Thompson, luckily though this is what led to Jett saving him.
ThatcherThatcher is more similar to Knuckles from Lippies V.3. He is an anthropomorphic short-beaked echidna, and the main character of C.I.O. Though rather than having terrible parents, Thatcher was raised in a "boarding school" (which was really an experiment facility hosted by Dr. Lee Thompson in disguise as a boarding school). Thatcher eventually met Finley, a shy but friendly red fox, who became Thatcher's first friend and later best friend. The two eventually escaped (one day they were going to experiment on Finley, Thatcher tried to stop but the building caught on fire due to an accident he caused) and five years after they first met, live in a house on the outskirts of a town.
While many characters in the Lippies trilogy follow naming patterns similar to the Sonic characters ([Name] the [Species]), Thatcher is simply referred to as "Thatcher" so that his name does not sound like that of a Sonic fan character. This is shared with Flint (formerly referred to as "Blast" or "Blaster" the Hedgehog), the hedgehog character that appeared alongside Clyde in Lippies V.3.
Thatcher is somewhat of a reference to how Knuckles was more supposed to be when he was first created. Since Knuckles is now shown to be retarded (especially in Sonic Boom) unlike before. Yeah I don't even know.