SillyTavern (a frontend with lots of features) (You need to provide your own backend or online service. Also all the settings can be a bit overwhelming for a beginner.)
KoboldCPP (easy to install, works without a high-end GPU) (Download models in GGUF format. Q4_K_M or Q5_K_M quantization is recommended.)
Guide (Don't pay too much attention to the old guides telling you to use a specific format. That has become obsolete. Listing properties or plain normal text is fine nowadays. But make sure to make it clear, concise and without contradictions. LLMs aren't too smart. And have a look at the prompt format of your model and recommended settings. That really matters.)
for anyone wanting to develop their own character cards, you can get a good base from teaching ChatGPT the W++ format, then hand-editing the spicy bits afterwords. I start by showing/pasting this page: https://rentry.co/WPP_For_Dummies