Complex sentence: While in Kyoto Keiji would meet Kanetsugu Naoe, Kagekatsu Uesugi's trusted assistant, and agrees to join Kanetsugu in his invasion of Aizu.
Split into simpler sentences: While in Kyoto Keiji met Kanetsugu Naoe, Kagekatsu Uesugi's trusted assistant. He then agreed to join Kanetsugu in his invasion of Aizu.