SHAPE:   It looked like two 
ΓΓ gamma letters, hence its name 'Digamma' (di- = twice + gamma).  Check variations of ancient 
cities of the 
Western-Eastern greek alphabet groups.
SOUND:  The indoeuropean semiphone /w/ appeared in greek as
1) vowel 
Y [u]  later pronounced [y]  and
2) consonant F [v]. [
v]  was a normal sound of ancient greek that was dropped in some 
dialects (
ionic/
attic - the eastern group) quite early, while in others (
doric - the  western group) it was still functional till 
hellenistic times.   It is said that a northsemitic letter: wa-w in its samaritan version influenced F [v] and by its phoenician sound influenced Y [u].
NAME: ETYMOLOGY/SPELLING:  Its hellenistic name when referring to its shape was 'Digamma':  di- = twice + 
gamma, because it looked like two 
ΓΓ letters.  For its sound, it is called [`vav] or [w`aw]. 
NUMBER: It would have represented number 6. Instead, six is represented either by the final 
Sigma ς or  sigma and tau  
στ´ which is called 
stigma στίγμα [`stiγma]
 
 
SHAPE: San looks like our own M.  The ancient Mu was a bit different.   Check variations of ancient 
cities of the 
Western-Eastern greek alphabet groups.   
SOUND:  San was used in some ancient cities to represent /s/ as a Sigma variation.  Soon it faded out, and 
Sigma Σ took over. 
NAME: ETYMOLOGY: Its name probably derives from the northsemitic (phoenician) letter: şādhē which sounded like /ts/.
 
It can be seen on early ancient epigraphs (inscriptions, curved texts).
SHAPE: Consistent in all 
cities of the 
Western-Eastern greek alphabet groups.
SOUND: The 
phoneme /k/ is uttered in greek in two ways:  [k] and 
palatalized [ç].  In the very early greek alphabets the Koppa was used to represent [k] and occurred before 
O /o/  and 
Y /y/ while Kappa K [ç] was used before /i/  /e/  and  /a/. Today, the two variants /k/ still exist.  Only difference is that /k/ becomes [ç] before /i/ and /e/ only.  
(We do not say [ça] anymore, but [ka].) .
NAME: ETYMOLOGY:  From ancient northsemitic (compare to hebrew: qōph).
NAME: SPELLING: It is written with double Pi.
AS NUMBER: The term 'koppa' survives at the greek numerical symbols with this different shape: 

´ = 90 or 
,
 = 90000.
SHAPE: NONE - a symbol was never created for this sound
SOUND:  We know that a /j/ sound was present in very early greek but no symbol is known from the ancient times.  Of course the sound exists today too.  It occurs in some 
Gamma Γ utterances and 
Iota I utterances. 
NAME:  Although there was no symbol that we know of, linguists named its imaginary symbol 
γιώτ, because /j/ sound was very active indeed:  it played a key role in many of the metamorphoses of many words.