Tobias first appeared in League Unleashed!, where he was obscured in shadows the entire time. Barry passed him in a hallway and was scared by the way he came off. In Casting a Paul on Barry!, while Ash and Barry were looking up information about their next League opponents, Conway told them that Tobias managed to easily obtain his eight Gym Badges and advance through the first two rounds of the Lily of the Valley Conference using just his Darkrai. Barry was amazed by his shadowy and mysterious presence.

In Working on a Right Move!, Tobias appeared unshadowed for the first time, and he was revealed to have advanced into the quarterfinals. In Battling a Thaw in Relations!, it was shown that he had advanced yet again, now moving onto the semifinals of the Lily of the Valley Conference.

In The Semi-Final Frontier!, he had a Full Battle against Ash in the semifinals. Although he was able to gain an early lead as his Darkrai alone defeated three of Ash's Pokémon, he eventually had to send out another Pokémon when Ash's Sceptile managed to overcome Darkrai's attacks and knock it out. Tobias then called out his Latios, who managed to defeat the remainder of Ash's team while fainting itself, winning the match for Tobias.

Later, Tobias defeated his final round opponent with just his Darkrai, thus winning the Lily of the Valley Conference and earning the right to participate in the Champion League. During the closing ceremonies, he was seen posing alongside Cynthia and Charles Goodshow while holding his trophy.

Tobias was a very mysterious person at first; his appearance and manner visibly disturbed Barry when they first passed each other in a hallway. He takes battling very seriously, but he also has a sportsmanlike side, as seen when he complimented Ash for defeating his Darkrai and putting up a good fight against him. Because of his serious dedication to battling, he could also become rather impatient when the end of a battle was near, such as when his Latios battled Ash's Pikachu.

Extra Info:

Tobias is one of the biggest mysteries in Pokémon the Series: Diamond and Pearl. The anime only officially revealed that he owned a Darkrai and a Latios, and he won the Sinnoh League before disappearing from the series.

Based on his mysterious appearance, overwhelming strength, and tendency to use rare Pokémon, here's a full six-Pokémon team that fits him well:

Tobias — The Phantom Champion

Region: Sinnoh

Title: Sinnoh League Champion

Battle Style: Overwhelming power, legendary Pokémon, calm and strategic.

1. Darkrai (Ace)

Type: Dark

Ability: Bad Dreams

Moves

Dark Void

Dark Pulse

Dream Eater

Ice Beam

The Pokémon that swept nearly the entire Sinnoh League by itself and defeated every Gym Leader Tobias faced.

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2. Latios

Type: Dragon/Psychic

Ability: Levitate

Moves

Luster Purge

Giga Impact

Dragon Pulse

Light Screen

The only other Pokémon officially seen in Tobias's team during his battle against Ash.

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3. Latias

Type: Dragon/Psychic

Ability: Levitate

Moves

Mist Ball

Psychic

Recover

Dragon Pulse

A natural partner to Latios. Many fans have long speculated that Tobias would own both members of the Eon Duo.

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4. Heatran

Type: Fire/Steel

Ability: Flash Fire

Moves

Magma Storm

Flash Cannon

Earth Power

Ancient Power

Provides excellent defensive coverage and fits Tobias's preference for powerful legendary Pokémon.

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5. Regigigas

Type: Normal

Ability: Slow Start

Moves

Crush Grip

Earthquake

Ice Punch

Zen Headbutt

A gigantic powerhouse capable of ending battles with raw strength.

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6. Cresselia

Type: Psychic

Ability: Levitate

Moves

Moonblast

Psychic

Lunar Dance

Ice Beam

The perfect counterpart to Darkrai. While Darkrai represents nightmares, Cresselia represents pleasant dreams.

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Tobias's Final Team

Pokémon Role

Darkrai Ace Sweeper

Latios Fast Special Attacker

Latias Support & Recovery

Heatran Defensive Tank

Regigigas Physical Powerhouse

Cresselia Support & Counter to Darkrai

Appearance Notes

From the image you shared, Tobias has:

Long dark-blue hair covering one eye

Pale blue eyes

A red cloak-like coat

A calm, intimidating expression

A mysterious "final boss" vibe that matches Darkrai perfectly

If Tobias ever returned in a modern Pokémon anime, this legendary-heavy roster would make him feel like an optional post-game boss that even Champions would be nervous to battle.