Who was Shanta?
Rishyasringa's wife.
Of course, king Romapada only brought Rishyasringa out of the forest and gave Shanta in marriage to him.
The question is whether Shanta is Romapada's own daughter or Dasharatha's biological daughter given in adoption to Romapada, his friend.
Let's see first the scriptural references to this.
Valmiki Ramayana has many versions.
If you see the southern version, it says Romapada is being advised, ‘Give your daughter Shanta in marriage to Rishyasringa.’
Then the wedding takes place yatha vidhi in the palace of Romapada.
Later, Dasharatha himself requests Romapada, ‘Please send YOUR daughter, tava sutaa,along with her husband to my place.’
And Dasharatha also says that I am childless, please send Shanta along with her husband, he should officiate my yaga to beget progeny.
Why would Dashratha call himself childless if Shanta is his daughter?
In the Gaudiya and Northwestern versions, it is said that Shanta is Romapda's own daughter.
In the Rama's story in m Mahabharata -
निवर्तितेषु सस्येषु यस्मै शान्तां ददौ नृपः
लोमपादो दुहितरं सावित्रीं सविता यथा
This is in VanaParva.
From MokshadharmaParva -
लोमपादश्च राजर्षिः शान्तां दत्त्वा सुतां प्रभुः
ऋश्यशृंगाय विपुलैः सर्वकामैरयुज्यत
Matsya Purana, 48th chapter says -
तस्य सत्यरथः पुत्रस्तस्माद्दशरथः किल
लोमपाद इति ख्यातस्तस्य शान्ता सुताभवत्
Romapada himself had another name, Dasharatha.
Dasharatha was his real name and he became famous as Lomapada or Romapada and his daughter was Shanta.
Harivamsa Purana also says the same.
Sarga 1, Chapter 31 -
अथ चित्ररथस्यापि पुत्रो दशरथोऽभवत्
लोमपाद इति ख्यातो यस्य शान्ता सुताभवत्
Dasharatha can be a title also.
someone whose chariot has gone in all ten directions, who has expanded his kingdom in all ten directions is a Dasharatha.
Like how we have Maharatha, Atiratha.
Or someone who has reined in all the ten senses, The five jnanaendriyas and five karmendriyas.
He can be a Dasharatha.
These are all references saying that Shanta is daughter of Lomapada or Romapada.
In the Gaudiya Patha of Valmiki Ramayana and also in the Paschima patha.
in the Udeechaya patha Romapada tells Rishyasringa -
अनेन मेऽनपत्याय दत्तेयं वरवर्णिनि
याचते पुत्रकृत्यार्थं शान्ता प्रियतमात्मजा
‘I was childless , he only gav me Shanta.’
Govindarja's commentary on Valmiki Ramayana, southern version takes the view that -
Here sumantra calls Rishyasringa as a son-in-law.
Whose son-in-law?
Govindaraja says, son-in-law of Romapada or Dasharatha.
Both are correct because Shanta is Dasharatha's biological daughter and given in adoption to Romapada.
The 11th sarga also says that Shanta is daughter of Dasharatha.
The 18th shloka of sarga 11 says about two kinds of relationship between Romapada and Dasharatha.
Not just friendship, sambadhaka also.
This is because of the adoption.
Srimad Bhagawatam, navama skandha, ch. 23 is very clear on this -
सुतो धर्मरथो यस्य जज्ञे चित्ररथोऽप्रजाः ।
रोमपाद इति ख्यातस्तस्मै दशरथः सखा ॥
शान्तां स्वकन्यां प्रायच्छदृष्यश‍ृङ्ग उवाह ताम् ।
देवेऽवर्षति यं रामा आनिन्युर्हरिणीसुतम् ॥
Dasharatha gave his own daughter Shanta to Romapada because Romapada was childless.
And she married Rishyasringa.
And you will see Dasharatha requesting Romapada, ‘Please send Shanta alone with her husband.’
Dasharatha is calling Rishyasringa as Shantabharta.
At the center of the relationship is Shanta, not just friendship with Romapada.
The obligation of Romapada towards Dasharatha is being reminded of again and again.
When Shanta comes to Ayodhya, all the ladies of the palace became happy to see Shanta having come with her husband.
if Shanta is just the wife of Rishyashringa who has come to just conduct a yaga, why should this reaction be there?
From all these, it is amply clear that Shanta was the biological daughter of Dasharatha and adopted daughter of Romapada.
Wherever it is mentioned that Shanta is the daughter of Romapada, of course, that is also true
because an adopted daughter is also a daughter.