An obituary for Finny?

This depends on what you wish to say. Try to capture the essence of the character. Finny was dynamic, reckless, charismatic....

The words should be few. Consider something like,

Phineas passed away after a short life. They say the brightest star burns half as long. Phineas shone as bright as they come. He was full of life....

That's a start but remember not to be too wordy.