To unlock the Sparrow before the end of the game, you have to get it through a Bright Engram. Bright Engrams are earned by leveling up. You can also purchase them with Silver, which is bought through microtransactions using real money. Turn the Bright Engrams into Tess Everis at The Farm to decrypt. There is a small chance you will unlock a Sparrow. To buy a Sparrow after completing the game, find Amanda Holliday in The Tower - Social Sector. She sells Sparrows. The Tower becomes available after completing the main campaign and defeating the final Boss. The Sparrow is a one-person mount a guardian can summon from their inventory to get around more quickly. Weapons cannot be used on Sparrows, but boosting into opponents will inflict damage. Your Sparrow cannot be destroyed. If you leave it behind, you can simply summon another one.