Knuckles the Echidna
Knuckles is the guardian of the Chaos Emeralds and has lived alone ever since he was forced to protect his charges for all eternity. Initially, Knuckles had no idea why he lived alone, why this was his responsiblity and where his people had gone. Instead, his first contact was with the villanous Doctor Robotnik, the tyrant tricking Knuckles that Sonic the Hedgehog was the real villain. The echidna believed Robotnik for some time, protecting him from Sonic and rescuing him from Kintobor's Laboratory, but eventually came to his senses when Robotnik absorbed the power of the Chaos Emeralds.
Ever since, Knuckles has lived a solitary life, only helping Sonic in his war if it directly affected the Emeralds or his island. For a while, he allowed the Emerald Hill folk to live in the Mushroom Hill Zone as a favour and made somewhat of a friendship with Porker Lewis, although the echidna was under no illusions that the responsibility of defending the island was his alone.
Knuckles' first meeting with another echidna (as far as he can remember) was Doctor Zachary, an elderly doctor being chased by one of the unknown Guardian Robots. Zachary pretended to be one of his people and Knuckles was keen to get information from him, until Zachary's robot destroyed the Master Emerald. The white echidna vowed to get "revenge" on the people of Mobius, being an "angel of death", but Knuckles stopped him and an earthquake-stricken Island threw Zachary to his death.
However, Zachary would return with Robotnik, now rebuilt with cyber technology after surviving his fall. The villain would assist Robotnik in his attack on the Emerald Hill folk, but when the plan failed, Zachary went into long hiding, only surfacing to attempt to control Super Sonic, before destroying the Special Zone (and nearly Mobius) under the guise of The Syndicate. Zachary also spent time as Knuckles' prisoner.
Tikal is an echidna from the ancient past with the amazing ability to see into the future. Not only that, but the seer has the ability to appear to those future people and bring them back to her time, something she did with Sonic. With the appearance of Chaos, Tikal was sealed away alongside the monster and would not resurface until thousands of years later thanks to a connection with Sonic. Her memories of her past were erased by Ebony to save from insanity and now lives with Knuckles and Porker.
The ruler of the Echidna empire was Pochacamac, an elderly echidna also with some power to see into the future. Pochacmac stole the Chaos Emerald from the Drakons before leading them into battle. It was Pochacamac's decision to send Sonic back into the future (inside Chaos' belly) and to make Knuckles become an eternal guardian.
One of a few echidna prisoners supposedly locked away deep in the Floating Island, Vichama was set free by Zachary in order to get some muscle in The Syndicate. Vichama went along with Zachary's plan, controlling his weapons and targetting them at Sonic and Tails. However, upon discovering that Zachary's ideas had failed, the "real" Vichama emerged from the dead echidna's body, using the power of the Chaos Emeralds to reveal himself to be the God of Death.
Echidnas in the ancient past would be initially viewed as primitive, using spears and coated in warpaint. Echidnas were actually technologically-advanced, already having built Guardian Robots thousands of years before they would be reactivated. It is unknown whether or not they won the Great War and if any others still exist today, but it is Knuckles and Tikal's goal to try and find a link to their past.