Print the Greek alphabet!

Ruby, 56 bytes

Full program. I don't think a function/lambda answer will be shorter than this for this language.

Dangit, sigma ς. This is why we can't have nice things. And you too, (unrenderable character that serves as a placeholder for "uppercase" ς)

print *($*[0]?([*?α..?ω]-[?ς]):[*?Α..?Ρ,*?Σ..?Ω])

05AB1E, 16 bytes

Uses CP-1252 encoding.

24Ý17K913+çJDl¹s

Try it online!

Explanation

24Ý               # push range [0 ... 24]
   17K            # remove 17
      913+        # add 913 to each
          ç       # convert from code point
           J      # join to string
            Dl    # push a lowercase copy
              ¹   # push the input
               s  # swap the top 2 elements of the stack
                  # implicitly display the top of the stack

JavaScript (ES6), 89 83 81 bytes

(...a)=>String.fromCharCode(...[...Array(24)].map((_,i)=>i*1.06+913+32*a.length))

If a character array is acceptable, then for 82 80 78 bytes:

(...a)=>[...Array(24)].map((_,i)=>String.fromCharCode(i*1.06+913+32*a.length))

Edit: Saved a bunch of bytes thanks to @ETHproductions.