After Stage 14 take the Gundam SEED route and then on Stage 15A have Shinn take Stella down to below 10% HP. When this task has been completed Stella will become available during the intermission before stage 32.
Have Kira attack the Legend Gundam on Stage 33 and then afterwards destroy it with Shinn. When this task has been completed Rey and the Legend Gundam will become available during the intermission after Stage 33.
The Pink Unicorn will appear when you complete Quickplay mode under the Hard difficulty setting without taking much damage. Accomplishing this will also unlock a trophy which if you tap multiple times during Quickplay mode will make the pink unicorn appear. This mythical beast is indestructible, kills everything, and has the ability to destroy the enemy castle in one hit.