Getting Started
First off, while playing Riku is almost the same as playing as Sora because their objectives are the same; you must clear floors by defeating the main boss and make it to the top floor. There is only one story room per floor, and fewer boss fights over all.
Undeniably the largest difference has to be the cards that Riku will use. As you'll probably notice right away when you try to reorganise your card deck, you cannot. Riku's deck is what is called a closed deck. Dumbed down that means; the deck is predetermined and cannot be altered, you'll just have to figure out how to maximize your deck with what you are given. Riku's cards are mostly attack cards. And he has but one weapon, the Soul Eater; all of his decks are based on and around that card.
In the beginning most of his decks have a single item card like Hi-Potion or Potion, only they won't be there forever. If you relied on magic through out Sora's game, abandon that idea, because no magic cards are used in Riku's deck. Start including enemy cards during battles.
Dark Mode
After defeating the first boss in Basement Twelve, Hollow Bastion, Ansem, Riku's nemesis, confronts you. Defeat him to learn how to use Riku's darkness in battle; Dark Mode. Dark Riku is similar in power to the Riku Replica you faced as Sora. When Riku is in Dark mode, his strength is increased but his speed is decreased, and now he can use sleights; Dark Break, Dark Firaga and Dark Aura.
Friends
Riku only gets to summon one friend, King Mickey. The King Card, which is different from the trick cards with the Mickey Mouse logo that you pick up in boss fights, summons King Mickey to the battle. Once on the battle field he restores HP, stuns and damages enemies and reloads all your cards. Stocking two or three of these cards increases the HP recovered and damage done.
Cards
Riku has fewer map cards than Sora. The maps he has access to are as follows:
Summary
Walkthrough
I'm really only going to go through each boss battle, seeing as it's not hard to go to the story rooms, that and because each floor is different based on what cards you use and even then, the floors will not be the same. I will however, give a brief summary of events leading up to each battle. I will also only give an explanation of the new bosses that Riku must fight, to defeat the other bosses, simply use tactics similar to those you used as Sora, their attacks and cards have not changed. Also, kill every Heartless you see.
Basement Twelve: Hollow Bastion
Main Events:
Boss: Ansem
This is an easy fight. You've got a well built deck and because it's so early in the game Ansem isn't going to throw his strongest attacks out at you. His shadow attacks on its own if you get too close. It shouldn't be too hard to card break every one of his attacks. The basics of card battling will make this an unbelievably easy battle. You have a choice of skipping all of the story rooms and head straight for the boss.
Ansem has two types of cards, Blue and Red. Blue controls his dark claw attack while Red triggers his shockwave. He has but one sleight this time around and is triggered by combining one Blue and one Red.
Blue Cards 0 1 2 3 4 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1 Red Cards 0 1 2 3 4 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1
Basement Eleven to Basement Eight
Several world cards become available at this time you must defeat the main boss for each one to continue to the next floor.
Main Events:
Boss: Vexen
You cannot hurt Vexen with front side attacks so I hope you took the time to become a master of Riku's dodge roll. If you haven't enter dark mode, target Vexen, jump and press "B" to land behind him. Vexen enters this battle with a new Sleight, Diamond Dust. He doesn't use it much but, this attack looks wonderful. It is made by combining a Red card and two Blue cards to perform. Just keep away from his Freeze attack, keep your health up, and just calm down and be patient as you slowly but surely kick his butt.
Blue Cards 0 1 2 3 4 5 6 7 8 9 3 2 1 0 0 0 0 1 2 3 Red Cards 0 1 2 3 4 5 6 7 8 9 3 2 1 0 0 0 0 1 2 3 Megalixir 0 1 2 3 4 5 6 7 8 9 0 0 0 0 1 1 0 0 0 0 Elixir 0 1 2 3 4 5 6 7 8 9 1 1 0 0 0 0 0 0 1 1 Enemy Cards Vexen, Blue Rhapsody, Air Pirate
Boss: Riku Replica
By now, you should know Riku's strengths and weaknesses as a character, which means you should exploit those same weaknesses that the Replica has. Go into Dark Mode immeadiately and use Sleight after Sleight and the Riku Replica should go down easy. He has three attack cards for three attacks. They'll be known as "Other Cards".The sleight Dark Firaga is made by combining two Blue cards and one Other Card.
Blue Cards 0 1 2 3 4 5 6 7 8 9 2 1 2 2 3 3 2 2 1 2 Red Cards 0 1 2 3 4 5 6 7 8 9 2 1 2 2 3 3 2 2 1 2 Other Attack Cards 0 1 2 3 4 5 6 7 8 9 0 2 0 0 2 3 2 0 2 0 Item Cards 0 1 2 3 4 5 6 7 8 9 0 0 0 0 1 1 1 0 0 0 Enemy Cards Shadow
Basement Seven to Basement Four
Main Events:
Boss: Lexaeus
Lexaeus likes earth magic. Before now, you haven't seen this in the game. His sleight, Rockshatter, which can take you by surprise if you aren't prepared, is triggered by combining two Blue cards with a Red card. He has three attacks so three attack cards. This isn't a tough fight, just use Dark Mode.
Blue Cards 0 1 2 3 4 5 6 7 8 9 2 2 2 2 2 2 2 2 2 2 Red Cards 0 1 2 3 4 5 6 7 8 9 0 0 0 3 3 3 3 3 3 0 Other Cards 0 1 2 3 4 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1 Enemy Cards Lexaeus
Basement Three: Destiny Island
Main Events:
Boss: Darkside
Yes, I know I said that I'd only do the bosses that are new to Riku mode but, I want to have a boss in each Basement so I have a reason to do all of them. He follows a set pattern of attacking. It's very vulnerable to face attacks when you can get to it, which sadly, isn't very often. Dark mode a sleights will beat him to a pulp, if you stay out of the way of his attacks.
Basement Two: Twilight Town
Main Events:
Boss: Riku Replica
By far, this is the toughest battle in Riku Mode. Riku Replica is just as fast and as strong as before. But, to make things worse, you're using the Twilight Town deck. You have no 0 cards, which is what makes this fight so damned tough. You don't even have King Mickey so, NO HEALING! Just go into Dark Mode as soon as possible and keep using sleights. This time around, Riku Replica has four different attacks. This fourth round of attack cards with herein be known as Attack Cards
Blue Cards 0 1 2 3 4 5 6 7 8 9 2 2 0 2 2 2 2 0 2 2 Red Cards 0 1 2 3 4 5 6 7 8 9 2 2 2 2 2 2 2 0 2 2 Other Cards 0 1 2 3 4 5 6 7 8 9 0 1 1 1 2 2 2 1 1 0 Attack Cards 0 1 2 3 4 5 6 7 8 9 1 2 2 0 0 0 0 2 2 2 Elixir 0 1 2 3 4 5 6 7 8 9 0 0 0 0 1 2 1 0 0 1 Enemy Cards Riku, Shadow
Basement One: Castle Oblivion
Main Events:
Boss: Ansem
Alright, boys and girls, remember how easy the first fight with Ansem was? Well forget it, because this time he's come prepared to battle. This time his shadow guards him from most front on physical attacks. As with every other boss battle, Dark Mode is the way to go.
Blue Cards 0 1 2 3 4 5 6 7 8 9 5 4 4 4 4 4 4 4 4 5 Red Cards 0 1 2 3 4 5 6 7 8 9 5 4 4 4 4 4 4 4 4 5 Elixir 0 1 2 3 4 5 6 7 8 9 0 0 0 0 0 0 0 0 0 2