Just after the tall vertical area with an elevator, go up the hole in the ceiling. It can get refilled by absorbing enemy projectiles, even if you don't equip the Hyper Cannon. It will automatically charge up your X-Buster with limited uses due to its ammunition meter. What it does: Adds Hyper Cannon to your arsenal.
After breaking the wall with Tornado Fang, climb up the wall to near the ceiling. It involves exploiting a glitch where changing to Zero in mid-air extends how far you can travel without dropping too much. There is however another, much easier, and much sneakier way to reach the capsule without the Leg Upgrade. Technically, it is possible reach the capsule without the Leg Upgrade, by dash jumping off of the wall underneath the overhang and reaching the platform leading to the capsule, however this is extremely difficult.
When you see a dark gray cracked wall, use the Tornado Fang to burst through the wall, get in the secret room, and use the Leg Upgrade to reach the capsule. You can also charge up the special weapons.