You don't show your flow, so I don't know exactly the type of approval you are using, but when the response comes back from the approval it includes this data:
"body": {
"responses": [
{
"responder": {
"id": "fa265a53-7380-4570-zzzzzzzzzzzzzzzzzz",
"displayName": "Doe, John",
"email": "john.doe@domain.com",
"tenantId": "e8d897a8-f400-4625-xxxxxxxxxxxxxxxxx",
"userPrincipalName": "john.doe@domain.com"
},
"requestDate": "2025-01-03T21:31:19Z",
"responseDate": "2025-01-04T15:22:51Z",
"approverResponse": "Approve"
}
],
"responseSummary": "Approver: Doe, John, john.doe@domain.com\r\nResponse: Approve\r\nRequest Date: Friday, January 3, 2025 9:31:19 PM\r\nResponse Date: Saturday, January 4, 2025 3:22:51 PM",
"completionDate": "2025-01-04T15:22:52Z",
"outcome": "Approve",
"name": "47148446-7082-xxxxxxxxxxxxxxxxx",
"title": "PR001632: DOA 3 Approval Required",
"details": "# You have an approval task. \n\nYou have been assigned a task to approve purchase requisition #001632 submitted by Jane Doe because you are the level 3 approver for the functional area Marketing Regional - Medical. Please click on the related item link to review the request. **Note:** the request will automatically be rejected if no response is received by **Friday, January 17, 2025**.\n\nIf you reject the request, please explain why it is being rejected in the comments field.",
"itemLink": "https://domaine.sharepoint.com/sites/sitename/_layouts/15/listform.aspx?PageType=4&ListId=c2cd3e93%2D1f0d%2D4c6dxxxxxxxxxxxxxx&ID=1632&ContentTypeID=0x01xxxxxxxxxxxxxxxBD710DD8AB2634BAC43260ED4631AB3",
"itemLinkDescription": "Related Item",
"requestDate": "2025-01-03T21:31:19Z",
"expirationDate": "9999-12-31T23:59:59Z",
"priority": "Medium"
}
}
You could use the displayName value depending on how they're configured in your environment and use an expression to extract just the first name. Or you could use the Get user profile action and pass the email value to it to return the user's first and last name values.
You will need to create a variable to store the value since the responses are in an array and will require an Apply to each action to extract the value. This is an example from my flow where I want to capture the comments:

Just use one of these two values depending on what you want to do: