One of the primary methods of acquiring companions is through the gacha; you can either consume five Transient Stones for a random character, or consume fifty for a multi-roll of ten. Multirolls increase the odds of acquiring a higher rarity unit.
Aside from the Main Gacha, event gachas offer modified drop/pull rates (event gachas offer characters with event-relevant skills at a higher drop rate), and sometimes gacha exclusives (Valentine's gacha offered exclusive variants of existing units).
Throughout the week, a gacha was released for one day to offer characters of a specific element (one timed-exclusive new, and others offered at a modified drop rate).