The Representative of Earth is in the Environment area and would be part of the Environment turn, so Legacy would bost damage until the start of the Environment turn. Any effects based on the Representative's turn will use the Environment turn.
The Representative of Earth is a Hero card, a hero target, a card from it's respective deck, a Hero Character card, and it is active.
It is not a Hero (requires a player and deck), it has no hero turn, no hand, no play area, no player. You can put cards next to it, but not "in its play area".
Thus:
Greatest Legacy, Prime Warden Fanatic, Argent Adept, Parse and Unity can not grant the Representative a power.
Vernal Sonata, Motivational charge, etc. would cause it to regain health.
Captain Cosmic could put any of his Constructs other than Crest next to the Representative, but only Autonomous Blade, Vitality Conduit, Wounding Buffer or Energy Bracers will do anything. The others all reference hero or player in the effect, and that will not apply to the Representative.
Fanatic could put Embolden on The Representative, but all it would do is let her deal it damage.
Guise can use its innate power with "I can do that too".
Flesh of the Sun God will make the Representative immune to Fire.
Proverbs and Axioms would allow the Representative to use a power, and to be considered the one using it. Which won't work for powers that have you play a card, draw a card, etc., but also would not cause it to take damage from Spite for PL626, because no hero used a power.
Things can get interesting, but just remember The Representative is not a Hero, because a Hero is a character played by a player, and the Representative has no player.
Called to Judgement would work as stated above, the Player that uses the power is the Hero, The reason Supertonic says a player may use a power is because that player will then get the effect of anything that says "you". This means if you got Called to Judgement against Spite with PL626 in play and you used the Representative's power, your Hero would take the damage and you would discard the top 5 of your deck.
Guise using another hero's power gets the benefit of anything that says "you".
Same for the Representative's power with Called to judgement.