Possible Clues:

• Letter increases capital 50% to secure a bit of interest
• 20th in a series
• Why shall we say it's a character from Athens?
• It's between tau and phi
• Greek character aloft, over lions set free
• A little retinol is purifying retired Greek character
• No lisp by upper class turns into a Greek letter
• Ancient character raised rural storage on end of plantation
• Y lookalike
• Erected missile site north in Europe, you say