Rielle gets +1/+0 for each instant and sorcery card in your graveyard.
Whenever you discard one or more cards for the first time each turn, draw that many cards.
Rielle turns discard into raw card advantage, then converts a stocked graveyard into both fuel for recursion and an oversized attacker. You spend the early game filtering with rummage and loot effects to fill the yard, then chain instants and sorceries with flashback, retrace, and replay effects to bury opponents in value or assemble a storm/spell-velocity kill.
Discard-to-draw means cards that normally cost you resources (rummaging, cycling, jump-start) net positive card advantage
Deep synergy with flashback, retrace, and aftermath spells that reuse cards from the graveyard
Rielle herself becomes a fast, cheap voltron threat once the graveyard fills with instants and sorceries
Excellent at refueling a hand after going low, making it hard to grind down
Izzet has premier card selection and burn to leverage all that draw
Heavily reliant on Rielle being on the battlefield to capitalize on discard, and she's a clear removal magnet
Gives all your graveyard instants and sorceries flashback for a value-laden or storm-style payoff turn.
Add more free or repeatable discard outlets (Anje's Ravager, Tortured Existence-style loops, Bone Miser-equivalents) and graveyard recursion (Snapcaster Mage, Mystic Retrieval, retrace cards like Raven's Crime) to maximize triggers. Tighten the mana base with fast rocks and dual lands, and slot a dedicated combo finish such as Underworld Breach plus Brain Freeze or Lion's Eye Diamond. Finally, run graveyard protection and stack interaction (Pyroblast, Swan Song) since Rielle and your yard are the deck's spine.