Mathias

Mathias was a member of the Brotherhood of Guardians, the father of Hawking and grandfather of Tobor, making him the oldest living Guardian by the time of Knuckles tenure as Guardian. Mathias served with the Brotherhood of Guardians most of his life, growing a strong hatred of the Dark Legion. As he got older however, Mathias left Haven and shed his corporeal form, establishing a closer link with the Chaos Force. He became quite popular among his bretheren, especially Locke. Mathias sacrificed his life in 3236 during the Brotherhood's attack on the Grand Conservatory, though evolved to an even high plane and joined the other deceased Brotherhood members.

Guardian
Little is known about Mathias' tenure as Guardian. What is known is that Mathias assisted with his son in dealing with the threat posed by the Dingoes. When Hawking was showing his video diaries of his days as Guardian to Knuckles, one of them showed him working along side Mathias. Mathias assisted Hawking in finding a means to prevent the destruction of Echidnaopolis, after the Dingoes threatened to destroy it with nuclear weapons. (KtE: #5)
 * It is presumed that the Guardian in the video recording is Mathias, as an "M" can be seen on his arm band. Additionally, he is the only other Guardian who would have been old enough at that time to be performing those duties.

Non-Coporeal Existance
As the years went by, Mathias became wise and eventually succeeded in ascending beyond his physical form, though he lost functionality of his eyes. Mathias eventually retired from his duties in Haven, and travelled to another dimension to spend the rest of his life in quiet meditation. (SSS: #14)
 * The exact amount of time Mathias spent in another dimension is unknown, as the concept of time had become irrelevant to him, though it was at least years as Locke recalled it had been so long since he was last in Haven.

Mathias was at peace with his surroundings, though his time there was brought to an end when suddenly a disturbance rippled across his dimension's plane of existence. Instinctively, Mathias knew only one being who would dare breach the fabric of existence, and he observed from his dimension that the being; Dimitri, preparing his forces for offensive against the Brotherhood and Echidnaopolis. (SSS: #14)

Return and Death
Mathias promptly returned to Haven, and upon reuniting with his bretheren, marshalled the Brotherhood for a preemptive strike against the Legion, as he sensed Dimitri would unleash his dark forces upon the innocent unless action was taken, and hoping whatever plans Dimitri was enacting would be thwarted. The Brotherhood unanimously agreed, and they teleported away. (SSS: #14)

Mathias traced the Legion to their most recent stronghold, and was shocked to find the Legion based at the Grand Conservatory, though Locke knew there was no better hiding place (as it was where the Echidna's had stored there renounced technology). The Brotherhood then launched an attack, with Spectre dealing with the sentries, and Mathias, after putting an end to a sudden burst of in-fighting and urging them to stay united, lead a meld of their powers with devastating effect, destroying Legion's weapon trucks and engulfing the base in flames. (SSS: #14)

Dimitri, while on the command centre of the Dark Legion Battle Cruiser, knew the Guardians were poised to interfere, was left with no choice but to unleash the Legion's ultimate weapon; the Quantum Beam Cannon, which caused the entire population of the Floating Island to be shifted to the Twilight Zone. All the members of the Brotherhood, including Mathias, were caught up by the Quantum Beam's effects. (SSS: #14)

Upon the Brotherhood's return, Locke was first to notice Mathias was no longer among them, but Spectre, possessing a higher level of omniscience, quickly learned of Mathias's fate (along with that of Hawking and Tobor; having died destroying the Quantum Beam shortly before the Brotherhood's return) and informed his comrades he had joined his decendants in "the next evolution". (StH: #107)

Strangely, Mathias was not seen amongst the other deceased Brotherhood members who spoke ot the deceased Knuckles while the latter watched his over funeral from the next plane of existence. (StH: #121)
 * It is possible Mathias had evolved to an even higher plane of existence like Athair, or he simply was not present at the time.