At the beginning of your end step, if Katara is tapped, put a +1/+1 counter on her.
Waterbend : Draw a card. (While paying a waterbend cost, you can tap your artifacts and creatures to help. Each one pays for .)
Katara is a mono-blue card-advantage engine: build a board of cheap creatures, tokens, and mana rocks, then tap them to repeatedly Waterbend and refill your hand. Tapping Katara herself each turn quietly grows her into a real threat, while the cards you draw fuel a controlling, value-driven game until you bury opponents in resources and close with a combo or a big Katara.
Repeatable, color-light card draw that scales with board width via the convoke-style waterbend cost
Mono-blue gives access to the best counterspells, card filtering, and tempo plays in the format
Doubles as a slow voltron: she gets bigger every turn she's tapped, even just from attacking or activating waterbend
Flexible identity — works as control, artifacts, or tokens without changing the core engine
Waterbend at is mana/board hungry; without bodies or rocks the engine stalls
Mono-blue lacks efficient removal and struggles against decks that go wide or pressure life totals
She is a high-priority removal target, and losing her cuts your draw engine
Tapping out your board for cards can leave you defenseless if you overcommit
Improvise turns artifacts into ramp, but more importantly tapping artifacts for value pairs with a wide rock-heavy board.
Add free or cheap untappers (Pemmin's Aura, Freed from the Real, Kiora's Follower) and mass-untap effects to convert Waterbend into a near-loop for explosive draws. Tighten the deck with premium counters and protection (Swan Song, An Offer You Can't Refuse, Heroic Intervention) plus a compact combo finish like Thassa's Oracle backed by Demonic Consultation-style self-mill. Finally, lower your curve with more one- and two-drop tappers and mana rocks so you can fire multiple Waterbends per turn well before the late game.