1) Go to Settings > General > Keyboards and enable an Asian language keyboard.

2) Open Notes and create a new note.

3) Tap the globe icon to change languages.

4) Tape the 123 key and enter a digit into your note.

5) Marvel as the font changes to Helvetica.

6) Tap the globe and change back to English.

That note will now be Helvetica permanently.

7) Cry about why this is not an option.