Sheena Fujibayashi is actually the character's alias and not her real name. This is because the people of Mizuho view their real names as sacred information. Therefore, only a handful of people - such as immediate family members - are allowed to know a Mizuho villager's real name.
Contributed by raidramon0
The Playstation 3 version of the game added a new set of costumes for everyone, putting them in the outfits of characters from other Tales games. These are unlocked by either beating the game and starting a New Game Plus file, or having save data from Tales of Graces f and/or Tales of Xillia. These are are:

Lloyd: Guy Cecil from Tales of the Abyss
Genis: Luke fon Fabre from Tales of the Abyss
Colette: Rita Mordio from Tales of Vesperia
Raine: Pascal from Tales of Graces
Sheena: Judith from Tales of Vesperia
Zelos: Alvin from Tales of Xillia
Regal: Malik Caesar from Tales of Graces
Presea: Elize Lutus from Tales of Xillia
Kratos: Ludger Will Kresnik from Tales of Xillia 2
Contributed by MightyKombat
In the GameCube version of the game, Lloyd, Genis and Colette possessed incredibly powerful secret techniques (nowadays called Mystic Artes) performed under specific circumstances.

Lloyd: Falcon's Crest
Genis: Indignation Judgment
Colette: Holy Judgment

The PlayStation 2 version of the game added Mystic Artes for the other playable characters as well as the bonus boss Abyssion (Who can use all the playable characters' Mystic Artes) and final boss Mithos. Some characters have 2 Mystic Artes as well. The PlayStation 2 version also added various miscellaneous artes for most of the cast. The (PlayStation 3) Chronicles rerelease of the game is based off of the PlayStation 2 version and retains the new moves.

Raine: Sacred Light (formerly unused in the GC version), Fairy Circle
Kratos, Zelos, Mithos, Abyssion: Shining Bind
Zelos' 2nd MA: Divine Judgment
Presea: Infernal Ruin, Crimson Devastation
Regal: Fanged Finality.

Sheena does not get any new Mystic Artes but her Summons now have a Mystic Arte character cutin image appear when performed.

Zelos as a playable character could now also learn and use his Judgment tech at any time once unlocked, Kratos gained a new healing/attack move called Guardian Field and Yggdrasil gained a move named "Yggdrasil Laser", a homage to Dhaos' infamous "Dhaos Laser" from Tales of Phantasia.
Contributed by MightyKombat
An unused tech for Raine, called "Sacred Light" can be found in the coding of the GameCube version of the game. The technique is almost fully complete, possessing a translated incantation ("Life, be thy sustenance and destroy the enemy!") and animation, though it does not damage the enemy.

For the PlayStation 2 version, Sacred Light was fully completed and implemented as Raine's first Mystic Arte, performed by using her Ray or Holy Lance spells 100 times then using anyone of them in Over Limit.

The English release of the PlayStation 3 version uses the formerly unused English incantation.
Contributed by MightyKombat
When Zelos wears his "formal" costume, there's a chance that during battle, his weapon will be changed to a bouquet of roses. This is a cosmetic change and offers no advantage or drawback.
Contributed by MightyKombat
The battle in Meltokio Colosseum features Spoiler:Garr from Tales of Destiny as well as Farah Oersted and Meredy from Tales of Eternia.

Originally, Spoiler:Rutee Katrea, also from Tales of Destiny, was meant to fight alongside the others as well. Her unused 3D model can still be found in the game's data.
Contributed by MightyKombat
Tales of Symphonia takes place thousands of years before the events of Tales of Phantasia, which is set in the combined worlds of Sylvarant and Tethe'alla, which is known by this point as Aselia.
Contributed by starwindbliss
After completing a short side-skit in the Lezereno Company with Presea in the lead of the party, she will obtain the 'Dream Traveller' title, which gives her an alternate costume. This costume is the same outfit that Klonoa wears.

Also, while Persea is in the outfit, various skits will unlock involving the outfit, which results in Persea saying Klonoa's famous phrase, 'Wa-hoo!', a lot.
Contributed by gaiages