From Scratchpad

To start the quest, talk to DB Okabe, who is by the Shinjuku exit to Nakano. He's riding a Cerberus, so you cannot miss him. Talk to him and answer Yes (はい) to start the process of gaining a mount.

You must now take this lectures on mounts. Select the first one and go through it. When you are done, you should return to normal view and an on-screen prompt should come up. Talk to Okabe again and answer Yes when he asks if you want to continue.

Do so and go through the second lecture option that has been added. Once done, you will get a Swift Cerberus and a test-drive ring. This is not the ring you want, so click on Okabe again and answer yes when he asks if you are done with the test-ride. Go through the final lecture and you will get a regular ring.

You are now done with the tutorial. Fron now on, you can get certain devils to be trained to be mounts. However, this training may actually be a trade for your current devil with a weaker mount version.

To use mounts, equip the ring item you received and have a mount Devil summoned. In your command window of the PC menu, you should have a purple command. While you are mounted you will move faster and be ignored by all devils. However, you cannot attack while mounted, and movement while mounted will decrease the durability of your rider ring.