When Sarah Potter went to the magical community they didn't understand why she wasn't ageing and after the second year in the magical community and how she was always attacked every year by different things which she deduced that it was Dumbledores plans to have her trained that she heard in on.
After that she ran to Japan using the money in her vault as a way to pay for many of her trips whilst there.
After she arrived she applied for the Navy School in Yokosuka where she met her good friend Chihaya Gunzō. She along with the others Sō, Kyōhei, Iori and Shizuka were good friends and did everything together and all got high marks, it was no surprise that they got to go on the trip to the secret facility housing the fog submarine.
(Sarah POV)
It seemed itself odd to me when the submarine started to turn on as soon as Chihaya touched the side of the submarine as reacting only to him and him alone. The tour guide had said that they had done many tests on the sub but it just wouldn't open up to them.
(Later that Week, Sarah POV)
We were all sitting together in the canteen when this tiny little blonde girl with quite long hair came up to the table and just pointed at Chihaya saying to meet him where his PDA had just shown him on a map but this was said to be a private meeting and Chihaya told us to let him go to it alone and he said he would message him if he needed us.
About two hours later Chihaya messaged us saying to meet us on the old fishing pier near the wall construction. When we got there about ten minutes with the girl and we knew that Chihaya hated time wasters and so we knew that this girl must have been worth his time so she must be worth his time. When he introduced her as Iona and that she had something to show us.
We all had the shock of our lives however when the same fleet of Fog submarine break the water behind them and hex steps appear in the air ready for us to take us up onto the ship. Chihaya said that Iona was in control of the ship but also wanted to have a strategic crew with the ability to remove the Fog from their seat of power in the oceans but he couldn't do it on their own so he had decided that he wanted us as his crew.