I’m no expert and I may be wrong, but I believe that when she said “ when I was thirteen, the fountain pen went with me to the Annex, and together we’ve RACED through countless diaries.” This is saying that the fountain pen raced through diaries which is something I’d say is definitely not possible for a pen to do. Why Anne uses this personification is because it gives us a better idea of how much she loved to use this pen that she wrote through many many diaries with it.