If your starting deck follows that rule, then the creature can serve as your chosen companion." The Wiki goes on to explain how you can only have one companion in a game, it starts outside the game, and counts towards your sideboard size (i.e., in Constructed play it is one of the fifteen cards in your sideboard)
For example, characters like Aang, Katara, and Sokka all have a card dedicated to them in one or two colors, gradually progressing to three or more colors
Available through The Planeswalker Championship Series (PWCS) tournaments in Japan, the three promo cards are available as follows: 1st place Basri Ket Top 8 Vito, Thorn of the Dusk Rose All participants Seasoned Hallowblade The PWCS started in the fall of 2019 and originally featured Vraska, Golgari Queen, Experimental Frenzy, and Pteramander as prizes
Once suspected, a creature remains suspected until it leaves the battlefield or another effect causes it to no longer be suspected