Type
  TMyKind = (mkA, mkB);

  [JsonSerialize(TJsonMemberSerialization.&In)]
  TBaseData = Class
    [JSonIn]
    [JsonName('Kind')]
    Kind: TMyKind;
    constructor Create; virtual;
  End;

  [JsonSerialize(TJsonMemberSerialization.&In)]
  TAData = Class(TBaseData)
    [JsonIn]
    Name: String;
    constructor Create; override;
  End;

  [JsonSerialize(TJsonMemberSerialization.&In)]
  TBData = Class(TBaseData)
    [JsonIn]
    Group: String;
    [JsonIn]
    Text: String;
    constructor Create; override;
  End;


  [JsonSerialize(TJsonMemberSerialization.&In)]
  TMyData = Class
    [JsonIn]
    [JsonObjectHandling(TJsonObjectHandling.Reuse)]
    Data: TArray<TBaseData>;

    [JsonIn]
    MainData: TBaseData;

    constructor Create; virtual;
    Destructor Destroy; override;
  End;


...

{ TMyData }

constructor TMyData.Create;
begin
  inherited;
  SetLength(Data, 2);
  Data[0] := TAData.Create;
  Data[1] := TBData.Create;
  MainData := TBData.Create;
end;

  fMyData := TMyData.Create;

  js := TJsonSerializer.Create;
  js.Formatting := TJsonFormatting.Indented;
  s := js.Serialize(fMyData);
  ...
  Memo.Text := s;

{
    "Data": [
        {
            "Name": "A",
            "Kind": 1
        },
        {
            "Group": "B",
            "Text": "C",
            "Kind": 0
        }
    ],
    "MainData": {
        "Group": "D",
        "Text": "E",
        "Kind": 1
    }
}

  LMyData := TMyData.Create;
  js := TJsonSerializer.Create;
  js.Populate<TMyData>(Memo.Text, LMyData);
  // LMyData.Data[0] = TBaseData
  // LMyData.Data[1] = TBaseData
  // LMyData.MainData = TBData
  ...
  Memo.Text := js.Serialize(LMyData)

{
    "Data": [
        {
            "Kind": 1
        },
        {
            "Kind": 0
        }
    ],
    "MainData": {
        "Group": "D",
        "Text": "E",
        "Kind": 1
    }
}